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Villén-Pérez, S; Carrascal, L.M. 2015. Occurrence data may provide unreliable thermal preferences and breadth of species. Current Zoology 61: 972–982. [pdf].
Carrascal, L.M.; Aragón, P.; Palomino, D.; Lobo, J.M. 2015. Predicting regional densities from bird occurrence data: validation and effects of species traits in a Macaronesian Island. Diversity and Distributions [pdf] [link]
Carrascal, L.M.; Galván, I.; Sánchez-Oliver, J.S.; Rey-Benayas, J.M. 2014. Regional distribution predicts bird occurrence in Mediterranean cropland afforestations. Ecological Research 29: 203–211. [pdf]
Seoane, J.; Villén-Pérez, S.; Carrascal, L.M. 2013. Environmental determinants of seasonal changes in bird diversity of Mediterranean oakwoods. Ecological Research 28:435-445. [pdf]
Villén-Pérez, S.; Carrascal, L.M.; Seoane, J. 2013. Foraging patch selection in winter: a balance between predation risk and thermoregulation costs. PLoS One 8(7): e68448. doi:10.1371/journal.pone.0068448 [pdf]
Carrascal, L.M.; Cayuela, L.; Palomino, D.; Seoane, J. 2012. What species-specific traits make a bird a better surrogate of native species richness? A test with insular avifauna. Biological Conservation 152:204-211. [SUMMARY] [pdf]
Carrascal, L.M.; Santos, T.; Tellería, J.L. 2012. Does day length affect winter bird distribution? Testing the role of an elusive variable. PLOS One 7(2): e32733. doi:10.1371/journal.pone.0032733. [SUMMARY] [link] [pdf]
Seoane, J.; Carrascal, L.M.; Palomino, D. 2011. Assessing the ecological basis of conservation priority lists for bird species in an island scenario. J. Nature Conservation 19:103-115. [SUMMARY] [pdf]
Rey Benayas, J.M.; Galván, I.; Carrascal, L.M. 2010. Differential effects of vegetation restoration in Mediterranean abandoned cropland by secondary succession and pine plantations on bird assemblages. Forest Ecology & Management 260:87-95 [SUMMARY] [pdf]
Carrascal, L.M.; Seoane, J.; Palomino, D.; Polo, V. 2008. Explanations for bird species range size: ecological correlates and phylogenetic effects in the Canary Islands. Journal of Biogeography 35:2061–2073 [SUMMARY] [pdf].
Seoane, J.; Carrascal, L.M. 2008. Interspecific differences in population trends of Spanish birds are related to habitat and climatic preferences. Global Ecology & Biogeography 17:111-121. [SUMMARY] [pdf]
Polo, V.; Carrascal, L.M.; Metcalfe, N.B. 2007. The effects of latitude and day length on fattening strategies of wintering coal tits (Periparus ater L): a field study and aviary experiment. Journal of Animal Ecology 76:866-872. [SUMMARY] [pdf]
Bosch, J.; Carrascal, L.M.; Durán, L.; Walker, S.; Fisher, M.C. 2007. Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link? Proceedings Royal Society London B 274:253–260 [SUMMARY] [pdf]
Seoane, J.; Carrascal, L.M.; Alonso, C.L. y Palomino, D. 2005. Species-specific traits associated to prediction errors in bird habitat suitability modelling. Ecological Modelling 185:299-308. [SUMMARY] [pdf].
Carrascal, L. M.; Lobo, J. L. 2003. Respuestas a viejas preguntas con nuevos datos: estudio de los patrones de distribución de la avifauna española y consecuencias para su conservación. Pp. 645-662 y 718-721 en Martí, R., Del Moral, J.C. (Eds.). Atlas de las Aves Reproductoras de España. Dirección General de la Conservación de la Naturaleza-Sociedad Española de Ornitología, Madrid. [pdf, Appendix] [link]
Belliure, J.; Carrascal. L.M. 2002. Influence of heat transmission mode on heating rates and on the selection of patches for heating in a Mediterranean lizard. Phys. & Bioch. Zool. 75(4):369-376. [SUMMARY] [pdf]
Polo, V.; Carrascal, L.M. 1999. Shaping the body size distribution of passeriformes: habitat use and body size are evolutionarily and ecologically related. Journal of Animal Ecology. 68:324-337. [SUMMARY] [pdf]
Carrascal, L.M.; Senar, J.C.; Mozetich, I.; Uribe, F.; Domenech, J. 1998. Interaction between environmental stress, body condition, nutritional status and dominance in mediterranean great tits (Parus major) during winter. Auk 115:727-738. [SUMMARY] [pdf]
Moreno, E.; Barbosa, A.; Carrascal, L.M. 1997. Should congruence between intra- and interspecific ecomorphological relationships be expected? A case study with the Great Tit Parus major. Proceedings of The Royal Society London: B 264:533-539. [SUMMARY] [pdf]
Carrascal, L.M.; Moreno, E.; Valido, A. 1994. Morphological evolution and changes in foraging behaviour of island and mainland populations of Blue Tit, Parus caeruleus. A test of convergence and ecomorphological hypoteses. Evolutionary Ecology 7:25-35. [SUMMARY] [pdf]
Tellería, J.L.; Carrascal, L.M. 1994. Weight-density relationships between and within bird communities. Implications of niche space and vegetation structure. American Naturalist 141:1083-1092. [SUMMARY] [pdf]
Carrascal, L.M.; Bautista, L.M.; Lázaro, E. 1993. Geographical variation in the density of the White Stork Ciconia ciconia in Spain: influence of habitat structure and climate. Biological Conservation 65:83-87. [SUMMARY] [pdf]
Carrascal, L.M.; Tellería, J.L.; Valido, A. 1992. Habitat distribution of canary chaffinches among islands: competitive exclusion or species-specific habitat preferences? Journal of Biogeography 19:383-390. [SUMMARY] [pdf]
Carrascal, L.M.; Cayuela, L.; Palomino, D.; Seoane, J. 2012. What species-specific traits make a bird a better surrogate of native species richness? A test with insular avifauna. Biological Conservation 152:204-211.
Identification of species-specific traits that make a species a better surrogate of biodiversity is a need in order to implement successful conservation programmes in the face of limited data and resources. This study analyzes the relationship between the abundance of different surrogate species and species richness for terrestrial native avifauna of autochthonous steppe and semiarid environments in Fuerteventura Island (Spain) at different spatial grains, and explores which species-specific ecological traits (body mass, ecological density, habitat breadth, coverage of urban and agricultural environments) and conservation features (endemicity, conservation status) make a species more efficient as a surrogate. Results indicate that abundance of those surrogate species which are typically targeted by local conservation managers (according to their rarity and increase public awareness) proves to be a poor predictor of three different measures of species richness of the native terrestrial avifauna of Fuerteventura at all spatial resolutions. Nonetheless, some species were found to perform better than others according to partial least squares regression analyses applied to relate species-specific ecological traits and conservation features with correlation coefficients between abundance of each bird species and total bird richness. The best surrogates for global bird species richness are those smaller birds of medium-high abundances, broad habitat preferences, less threatened status, and with a high degree of endemicity. No scale-dependency was observed in the surrogacy power of species. Conservation planners in island scenarios should use a selection of bird species with these characteristics to identify conservation target areas in order to maximize the efficiency of surrogacy approaches.
Carrascal, L.M.; Santos, T.; Tellería, J.L. 2012. Does day length affect winter bird distribution? Testing the role of an elusive variable. PLoS One 7(2): e32733. doi:10.1371/journal.pone.0032733.
Differences in day length may act as a critical factor in bird biology by introducing time constraints in energy acquisition during winter. Thus, differences in day length might operate as a main determinant of bird abundance along latitudinal gradients. This work examines the influence of day length on the abundance of wintering crested tits (Lophophanes cristatus) in 26 localities of Spanish juniper (Juniperus thurifera) dwarf woodlands (average height of 5 m) located along a latitudinal gradient in the Spanish highlands, while controlling for the influence of food availability, minimum night temperature, habitat structure and landscape characteristics. Top regression models in the AIC framework explained 56% of variance in bird numbers. All models incorporated day length as the variable with the highest magnitude effect. Food availability also played an important role, although only the crop of ripe juniper fruits, but not arthropods, positively affected crested tit abundance. Differences in vegetation structure across localities had also a strong positive effect (average tree height and juniper tree density). Geographical variation in night temperature had no influence on crested tit distribution, despite the low winter temperatures reached in these dwarf forests. This paper demonstrates for the first time that winter bird abundance increases with day length after controlling for the effect of other environmental variables. Winter average difference in day length was only 10.5 minutes per day along the 1º47’ latitudinal interval (190 km) included in this study. This amount of time, which reaches 13.5 h accumulated throughout the winter season, appears to be large enough to affect the long-term energy budget of small passerines during winter and to shape the distribution of winter bird abundance under restrictive environmental conditions.
Seoane, J.; Carrascal, L.M.; Palomino, D. 2011. Assessing the ecological basis of conservation priority lists for bird species in an island scenario. J. Nature Conservation 19:103-115.
Proneness to extinction varies naturally and continuously according to the ecological phenomena that compound rarity even before anthropogenic effects may play a role. This is particularly obvious in islands, where populations are often (and naturally) small and fragmented and, consequently, conservation priority lists may have a large number of species clustered unhelpfully in the higher threat categories. In this study we propose a simple model of threat based on natural descriptors of rarity and taxonomic distinctiveness (area of occupancy, population abundance and trend, and endemicity), assess its correlation with ecological features of the species (habitat preferences and body size) and check whether the Spanish Red data Book and a normative conservation priority list (the Canary Islands Catalogue of Threatened Species and its administrative revision) includes these ecological bases for birds. We found that a large variation in threat (48.2%) was explained by phylogeny, habitat breadth and preference for urban areas (with a negative effect), and preference for agricultural environments (a positive effect). The Spanish Red data Book and the administrative lists tested are poorly related to descriptors ordering the extinction risk and loss of taxonomic singularity, so some changes would make their categories more coherent. We contend that the ecological bases of rarity should be taken into account to understand why some populations/ species are at higher extinction risk whereas others remain relatively safe, as this would provide firmer grounds on which to base conservation priorities.
Rey Benayas, J.M.; Galván, I.; Carrascal, L.M. 2010. Differential effects of vegetation restoration in Mediterranean abandoned cropland by secondary succession and pine plantations on bird assemblages. Forest Ecology & Management 260:87-95.
Two contrasting trajectories for vegetation restoration in agricultural landscapes are secondary succession following cropland abandonment that can regenerate woodlands (passive restoration) and conversion of cropland to tree plantations (active restoration), which have mostly focused on pine species in the Mediterranean Basin. We compared the effects of these two contrasting trajectories of vegetation restoration on bird assemblages in central Spain. Vegetation structure differed in the two restoration trajectories, pine plantations attaining higher tree cover and height (31% and 4.1 m, respectively) but lower strata complexity than secondary shrubland and holm oak woodland (which attained 10% and 1.4m of tree cover and height, respectively). Bird species richness differed in stands under active or passive restoration trajectories, the former collecting a higher total number of species (4.2 species per 0.78 ha plot) than the latter (3.5 species per plot). The number of forest species increased with vegetation maturity in both restoration trajectories, but especially in stands under active restoration. The occurrence of woodland generalist species increased and of species inhabiting open habitats decreased in actively restored stands, being some of these latter species of high conservation priority in the European context but relatively common at the regional level. Bird species inhabiting pine plantations had broader habitat breadth at the regional level than those inhabiting secondary shrublands and woodlands. Maximum regional density did not differ between both restoration trajectories, but it increased with development of the herbaceous layer only at the secondary succession trajectory. The relative importance of species of European biogeographic origin was higher in mature pine plantations (58.9% of total bird abundance) than in mature holm oak woodlands (34.4%), whereas that of Mediterranean species was considerably higher in the latter (40.1%) than in the former (20%). Bird assemblages of relatively small patches of pine plantations are unable to reflect the regional avifauna, in contrast with the relationships between local and regional assemblage characteristics that can be found in isolated natural forests. We conclude that programs of vegetation restoration should base upon a range of approaches that include passive restoration, active restoration with a variety of tree and shrub species, and mixed models to conciliate agricultural production, vegetation restoration and conservation of target species.
Carrascal, L.M.; Galván, I.; Gordo, O. (accepted). Partial least squares regression as an alternative to most currently used regression methods in Ecology. Oikos, 118:681-690.
This paper briefly presents the aims, requirements and results of partial least squares regression analysis (PLSR), and its potential utility in ecological studies. This statistical technique is particularly well suited to analyzing a large array of related predictor variables (i.e., not truly independent), with a sample size not large enough compared to the number of independent variables, and in cases in which an attempt is made to approach complex phenomena or syndromes that must be defined as a combination of several variables obtained independently. A simulation experiment is carried out to compare this technique with multiple regression (MR) and with a combination of principal component analysis and multiple regression (PCA+MR), varying the number of predictor variables and sample sizes. PLSR models explained a similar amount of variance to those results obtained by MR and PCA+MR. However, PLSR was more reliable than other techniques when identifying relevant variables and their magnitudes of influence, especially in cases of small sample size and low tolerance. Finally, we present one example of PLSR to illustrate its application and interpretation in ecology.
Carrascal, L.M.; Seoane, J.; Palomino, D.; Polo, V. 2008. Explanations for bird species range size: ecological correlates and phylogenetic effects in the Canary Islands. Journal of Biogeography 35:2061–2073.
Aim To explore the determinants of island occupancy of 48 terrestrial bird species in an oceanic archipelago, accounting for ecological components while controlling for phylogenetic effects.
Location The seven main islands of the Canary archipelago.
Methods We obtained field data on population density, habitat breadth and landscape distribution in Tenerife, Fuerteventura and La Palma, aiming to sample all available habitats and the gradient of altitudes. In total, 1715 line transects of 0.5 km were carried out during the breeding season. We also reviewed the literature for data on occupancy, the distance between the Canary Islands and the nearest distribution border on the mainland, body size and endemicity of the 48 terrestrial bird species studied. Phylogenetic eigenvector regression was used to quantify (and to control for) the amount of phylogenetic signal.
Results The two measurements of occupancy (number of occupied islands or 10 · 10 km UTM squares) were tightly correlated and produced very similar results. The occupancy of the terrestrial birds of the Canary Islands during the breeding season had a very low phylogenetic effect. Species with broader habitat breadth, stronger preferences for urban environments, smaller body size, and a lower degree of endemicity had a broader geographical distribution in the archipelago, occupying a larger number of islands and 10 · 10 UTM squares.
Main conclusions The habitat-generalist species with a tolerance for novel urban environments tend to be present on more islands and to occupy a greater area, whereas large-sized species that are genetically differentiated within the islands are less widespread. Therefore, some properties of the ranges of these species are explicable from basic biological features. A positive relationship of range size with local abundance, previously shown in continental studies, was not found, probably because it relies on free dispersal on continuous landmasses, which may not be applicable on oceanic islands.
Palomino, D.; Carrascal, L.M. 2007. Threshold distances to nearby cities and roads influence the bird community of a mosaic landscape. Biological Conservation 140:100-109.
Urban developments and road networks extend their impacts on the surrounding habitats along a variable distance, affecting birds living in natural environments. This study identifies the threshold distances upon which several cities and roads, located across a large mosaic landscape of ca. 300 km2 in central Spain, alter the abundance patterns of the native avifauna. Total species richness, total bird abundance, and abundance of different guilds of birds which are potentially sensitive to human disturbances were modelled by means of tree regression analyzes. Nearby cities do not affect the total bird species richness in natural habitats of the study region. Total bird abundance increases near urban areas, mainly through its positive influence on urban-exploiter species. The effect of roads is negative and highly generalized, although threshold distances to roads vary among different groups of species. The bird communities of deciduous woodlands (ash groves, oak patches and poplars) show higher resilience to deletereous influences from nearby cities and roads. It would be desirable not to build new scattered urban developments within the remnant natural areas of this heavily fragmented region, because their existence and connection to the nearby cities by new roads would add ‘invisible’ negative effects on the native bird fauna (e.g. on some threatened species from open habitats), considering the buffer distances determining most significant impacts (400m for urban areas, and 300m for roads).
Seoane, J.; Carrascal, L.M. (2007). Interspecific differences in population trends of Spanish birds are related to habitat and climatic preferences. Global Ecology & Biogeography 17:111-121.
1. Aim Animal monitoring programs have allowed analyses of population trends, most of which recently comment on the possible effect of global climate change. However, the relationship between the interspecific variation in population trends and species’ traits such as habitat preferences, niche breadth or distribution patterns have received little attention, in spite of its usefulness in the construction of ecological generalizations. The objectives of this study were to (1) determine whether there are characteristics shared among species with upwards or downwards trends, and (2) assess whether population changes agree with what could be expected under global warming (a decrease of species typical of cooler environments).
2. Location The Spanish part of the Iberian Peninsula (ca. 500,000 km2) in the south-western part of the Mediterranean Basin.
3. Methods We modelled recent breeding population changes (1996-2004), in areas without aparent land use changes, for fifty-seven common passerine birds with species-specific ecological and distributional patterns as explanatory variables.
4. Results One-half of these species have shown a generalized pattern towards the increase of their populations, while only one-tenth showed a significant decrease. One half (54%) of the interspecific variability in yearly population trends is explained considering species-specific traits. Species showing more marked increases preferred wooded habitats, were habitat generalists and occupied warmer and wetter areas, while moderate decreases were found for open country habitats living in drier areas.
5. Main conclusions The coherent pattern in population trends we found disagree with the proposed detrimental effect of global warming on bird populations of Western Europe, which is expected to be more intense in bird species inhabiting cooler areas and habitats. Such pattern suggests that factors other than the increase in temperature may be brought to discussions on global change as relevant components to explain recent changes in biodiversity.
Polo, V.; Carrascal, L.M.; Metcalfe, N.B. (2007). The effects of latitude and day length on fattening strategies of wintering coal tits (Periparus ater L): a field study and aviary experiment. Journal of Animal Ecology, 76:866-872.
1. Cyclic daily fattening routines are very common in wintering small wild birds, and are thought to be the consequence of a trade-off between different environmental and intrinsic factors. According to theory, these trajectories should range from accelerated (i.e. mass increases exponentially towards dusk) when mass-dependent costs are the most important cause of mortality, to decelerated (i.e. the rate of mass gain is highest at dawn and decreases afterward) when starvation is the greater risk.
2. We examine if geographically separate populations of coal tits, wintering in Scotland and Central Spain under contrasting photoperiods, show differences in their strategies of daily mass regulation. We describe population differences in wild birds under natural conditions, and experimentally search for intrinsic interpopulation variation in diurnal body mass increase under common, manipulated, photoperiod conditions (9 h Light:15 h Dark vs 7L:17D) , controlling for temperature, food availability, predator pressure and foraging arena.
3. Winter diurnal mass gain of wild coal tits was more delayed towards the latter part of the daylight period in Central Spain (i.e., the locality with longer winter days) than in Scotland. In both localities, the pattern was linked to the average mass at dawn, with mass increasing more rapidly in lighter birds. However, under the controlled photoperiod situation the pattern of daily mass gain was similar in both populations. Diurnal body mass gain was more accelerated at the end of the day, and the increase in body mass in the first hour of the day was considerably lower under the long (9 hours) than under the short (7 hours) photoperiod in both populations.
4. Wintering coal tits show patterns of mass gain through the day that are compatible with current theories of the costs and benefits of fat storage, with birds at lower latitudes (with longer winter days) having a greater tendency to delay mass gain until late in the day. The experimental study revealed that these patterns are plastic, with birds responding directly to the photoperiod that they experience, suggesting that they are continually making fine-scale adjustments to energy reserves on the basis of both intrinsic (e.g. state-dependent) and extrinsic cues.
Bosch, J.; Carrascal, L.M.; Durán, L.; Walker, S.; Fisher, M.C. (2007). Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link? Proceedings Royal Society London B, 274:253–260.
Amphibian species are declining at an alarming rate on a global scale in large part due to an infectious disease caused by the chytridiomycete fungus, Batrachochytrium dendrobatidis. This pathogen of amphibians has recently emerged within Europe, but knowledge of its widespread effects on amphibian assemblages remains poor. Importantly, little is known about the environmental envelope that is associated with chytridiomycosis in Europe, and the potential for climate change to drive future disease dynamics. Here, we use long-term observations on amphibian population dynamics in the Peñalara Natural Park, Spain, to investigate the link between climate-change and chytridiomycosis. Our analysis shows a significant association between local climatic variables and the occurrence of chytridiomycosis within this region. Specifically, we show that rising temperature and humidity are linked to the occurrence of chytrid-related disease, and that these local variables are driven by general circulation patterns, such as the North Atlantic oscillation. Given that the chytrid is known to be broadly distributed across Europe, there is now an urgent need to assess how climate-driven epidemics are expected to impact on amphibian species across the wider region.
Carrascal, L.M.; Díaz, L. 2006. Winter bird distribution in abiotic and habitat structural gradients. A case study with mediterranean montane oakwoods. EcoScience 13:100-110.
The influence of habitat structure and abiotic factors on winter bird distribution was studied at the within-habitat level in the montane Pyrenean oakwoods of central Spain. Abiotic factors associated with thermal stress were estimated based on altitude and solar radiation received by woodlands (calculated by the steepness and orientation of the terrain). This paper demonstrates the great importance of abiotic factors in influencing bird distribution. Several bird community parameters related to density and species richness decreased with altitude, while they increased with radiation incidence of oakwood plots (i.e., birds avoided northern orientations where solar radiation is minimal in winter). The most important habitat structure variables related to bird distribution were the density of young and mature oaks. A thick undergrowth of thin oaks negatively influenced total bird abundance and species richness and the number of species of the ground searchers guild. Conversely, oak maturity played a positive role on total bird density and species richness and on the number of species of tree canopy gleaners and trunk foragers. Bird density and species richness were better explained by tree regression models considering complex interactions between variables than by general linear regression analyses. To enhance winter survival and habitat suitability for birds, forest management in these mediterranean endemic oakwoods should preserve the most mature forests at lower altitudes exposed to the south.
Palomino, D.; Carrascal, L.M. 2006. Urban influence on birds at a regional escale. A case study with the avifauna of northern Madrid province. Landscape & Urban Planning 77:276-290.
Bird fauna of the Madrid province (Central Spain) was analyzed according to urban development in a landscape mosaic of 700 km2. Bird distribution and abundance was studied in urban versus several rural habitats and along a gradient of urban typologies. By means of tree regression analyses we identified the most important habitat structure variables affecting bird species richness and density in urban environments. Bird communities in urban environments were globally less diverse and had higher densities than any natural habitat of the study region. The number of urban-avoider species (n = 37) was greater than the number of species favoured by urban habitats (n = 8). Current housing developments of extense crowded terraced-houses, with shortage of gardens, supported the least diverse and dense bird populations. Nevertheless, differences in bird species abundance between urban and natural habitats mitigated in many species when considering the older gardened developments. The plots with the highest species richness (average of 14.5 spp./0.8 ha) were those with 15–28% of building cover, more than 43 medium-sized trees/ha (10–30 cm dbh), and 13–54 small trees/ha (less than 10 cm dbh). Subsequently, future land-use planning should stress the exclusion of urban developments from the most valuable habitats, such as open wooded valley areas devoted to cattle-grazing (mainly ash-groves), and the negative effect of dense, low-gardened housing developments.
De La Montaña, E.; Rey Benayas, J.M.; Carrascal, L.M. 2006. Response of bird communities to silvicultural thinning of Mediterranean maquis. Journal of Applied Ecology 43:651–659.
1. Land owners in some European Mediterranean regions receive subsidies to thin dense maquis. This practice consists in the elimination of most shrubs and saplings and the pruning of the tallest trees to favour more opened woodland stands. We investigated how this practice affects the structure of bird communities. 2. We designed a large scale ‘natural experiment’ that included 21 paired thinned and un-thinned maquis stands in Central Spain. Every stand was sampled by means of 5 point counts, each consisting of a 50-m radius plot, in two consecutive years and in winter and spring. The vegetation structure was characterized after bird censuses in 10-m radius plots that coincided with the centers of the bird point counts. Data analyses were based on repeated-measures ANOVAs. 3. Thinning was responsible for a significant increase in species richness, but did not have any effect on total bird density. Average body mass of species in thinned stands was significantly larger than in un-thinned, more densely vegetated, stands. Density of ground searchers was undistinguishable in thinned and in un-thinned stands, whereas density of foliage gleaners was higher in un-thinned stands. Winter density of granivorous species was marginally higher in thinned stands, whereas insectivorous and frugivorous species were marginally more abundant in un-thinned stands. 4. Thinned areas allow the occupation with higher densities of bird species whose European conservation status is of higher concern. Winter density of game birds was higher in thinned stands. 5. Synthesis and application. This is the first time that a large-scale experimental manipulation of habitat structure and vegetation volume demonstrates the predicted allometric effect of habitat structural complexity on the average body mass of a bird community. Thinning of dense Mediterranean woodland enhances habitat heterogeneity and suitability for several bird species and increased species richness. This practice was also beneficial for species of conservation concern and the hunting of non-threatened game birds. However, some un-thinned patches should be preserved to provide refuge for the few species that are impacted by thinning.
Seoane, J.; Carrascal, L.M.; Alonso, C.L. y Palomino, D. (2005). Species-specific traits associated to prediction errors in bird habitat suitability modelling. Ecological Modelling 185:299-308.
Although there is a wide range of empirical models applied to predict the distribution and abundance of organisms, we lack an understanding of which ecological characteristics of the species being predicted affect the accuracy of those models. However, if we knew the effect of specific traits on modelling results, we could both improve the sampling design for particular species and properly judge model performance. In this study, we first model spatial variation in winter bird density in a large region (Central Spain) applying regression trees to 64 species. Then we associate model accuracy to characteristics of species describing their habitat selection, environmental specialization, maximum densities in the study region, gregariousness, detectability and body size.Predictive power of models covaried with model characteristics (i.e., sample size) and autoecological traits of species, with 48% of interspecific variability being explained by two partial least regression components. There are species-specific characteristics constraining abundance forecasting that are rooted in the natural history of organisms. Controlling for the positive effect of prevalence, the better predicted species had high environmental specialization and reached higher maximum densities. We also detected a measurable positive effect of species detectability. Thus, generalist species and those locally scarce and inconspicuous are unlikely to be modelled with great accuracy. Our results suggest that the limitations caused by those species-specific traits associated with survey work (e.g., conspicuousness, gregariousness or maximum ecological densities) will be difficult to circumvent by either statistical approaches or increasing sampling effort while recording biodiversity in extensive programs.
Carrascal, L.M.; Polo, V. (2006). Effects of wing area reduction on body mass and foraging behaviour in the coal tit (Parus ater) during winter: field and aviary experiments. Animal Behaviour 72: 663-672.
Theoretical and experimental evidence suggests that an increase in flight costs will cause a decrease in flight performance, and that birds should trade-off the benefits of body reserves to minimize these costs. However, an alternative strategy could be to avoid the dangers of starvation by increasing food intake, thereby maintaining body reserves, and/or decreasing flight activity to compensate for the greater per unit flight costs. To test the effect of increased flight costs on body mass regulation and on flying and feeding activity, we experimentally manipulated wing area in a free-ranging wintering population of coal tits (Parus ater), and in captive birds living in a less restrictive environment (large outdoor aviaries). In the field, there was a clear trend towards body mass decrease when wing area was experimentally reduced, although it was not homogeneous: heavier birds lost more weight than lighter birds as a consequence of an allometric increase in flying costs. However, the experimental reduction of wing area had a non-significant, negligible, effect on body mass in the aviaries. Flight and feeding frequency were significantly affected by the experimental reduction of wing area: birds flew less and ate more when wing area was reduced. Birds with higher wing loads decreased more markedly flying frequency when wing area was reduced. We suggest that the goal of small resident birds living in Mediterranean montane climate would be to maintain daily fat reserves within some narrow limits during autumn and winter, even under contrasting ecological conditions. Our results show a trade-off between wing area reduction and body mass (in the field) and a trade-off between wing area and flying frequency (in the aviaries), both mediated by allometric effects of body size.
Carrascal, L.M.; Alonso, C.L. (2005). Habitat use under latent predation risk. A case study with wintering forest birds. Oikos 112: 51-62
We test the prediction that predation risk is a foraging cost affecting the spatial distribution of birds within habitat. The work was carried out in a montane mixed forest of Central Spain with four Parus species, the Long-tailed tit (Aegithalos caudatus) and the Nuthatch (Sitta europaea). Using specially designed feeders containing the same amount and kind of food, we control for interspecific differences in food preferences and in foraging postures related to ecomorphological constraints, and for differences in natural food availability among foraging substrata. Small tree gleaning passerines avoided feeding on dark inner forest places far from edges, distant from protective cover, outside the tree canopy and near the ground; they preferred deciduous, relatively clear forest plots. These effects remained invariable across years and weather conditions. There was a common pattern of selection of foraging locations by the four Parus species: distance to cover (negatively), and height above ground and over the lowest branches of the tree canopy (positively) markedly determined the use of feeding places. According to these patterns, the vigilance proportion of species was significantly higher when feeding far from cover than when birds were feeding near pine foliage. This pattern was also common for the four studied Parus species. Nevertheless, the interspecific dominance hierarchy of the species was negatively correlated with the use of the most exposed feeders (feeders nearer the ground and more distant from cover and below the lower branches of tree canopy), being the converse with the safest ones. Therefore, the results of this paper demonstrate that the selection of feeding locations within habitat follows a pattern minimizing predation risk. Interspecific dominance hyerarchies can lead to the exploitation of unfavourable risky patches by subordinate species.
BELLIURE, J.; CARRASCAL. L.M. (2002). Influence of heat transmission mode on heating rates and on the selection of patches for heating in a Mediterranean lizard. Phys. & Bioch. Zool. 75(4):369-376.
Heliothermy (heat gain by radiation) has been given a prominent role in basking lizards. However, thigmothermy (heat gain by conduction) could be relevant for heating in small lizards. To ascertain the importance of the different heat transmission modes to the thermoregulatory processes, we conducted an experimental study where we analyzed the role of heat transmission modes on heating rates and on the selection of sites for heating in the Mediterranean lizard Acanthodactylus erythrurus (Lacertidae). The study was conducted under laboratory conditions, where two situations of different operative temperatures (38° and 50°C) were simulated in a terrarium. In a first experiment, individuals were allowed to heat up during 2 min at both temperatures and under both heat transmission modes. In a second experiment, individuals were allowed to select between patches differing in the main transmission mode, at both temperatures, to heat up. Experiences were conducted with live, nontethered lizards with a starting body temperature of 27°C. Temperature had a significant effect on the heating rate, with heat gain per unit of time being faster at the higher operative temperature (50°C). The effect of the mode of heat transmission on the heating rate was also significant: at 50°C, heating rate was greater when the main heat transmission mode was conduction from the substrate (thigmothermy) than when heating was mainly due to heat gain by radiation (heliothermy); at 38°C, heating rates did not significantly differ between transmission modes. At 38°C, selection of the site for heating was not significantly different from that expected by chance. However, at 50°C, the heating site offering the slowest heating rate (heliothermic patch) was selected. These results show that heating rates vary not only with environmental temperature but also with different predominant heat transmission modes. Lizards are able to identify and exploit this heterogeneity, selecting the source of heat gain (radiation) that minimizes the risk of overheating when temperature is high.
CARRASCAL, L.M.; DÍAZ, J.A.; HUERTAS, D.L.; MOZETICH, I. 2001. Behavioral thermoregulation by treecreepers: trade-off between saving energy and reducing crypsis. Ecology 82:1642-1654.
We studied the effect of solar radiation on the winter biology of Short-toed Treecreepers Certhia brachydactyla inhabiting a montane coniferous forest. We hypothesized that, in temperate latitudes of cold winter climate with low cloudiness and under windless conditions, birds should select sunlit sites (i.e., forest sectors or trunk patches with high levels of exposure to sunlight) to reduce the metabolic cost of thermoregulation. At a within-habitat scale, a hypothesis of "only metabolic benefits" predicts that birds should select sunlit patches at temperatures in the shade (Tshade) below the lower critical temperature (Tlc), and shift to a random use of sunlit and shaded patches at temperatures above Tlc. Alternatively, there could be added costs (e.g. travel costs, predation risk) to the use of sunlit patches. If higher visibility leads to diminished crypsis at sunlit patches ("trade-off with predation risk hypothesis"), birds should select only shaded patches at Tshade values above Tlc (to enhance crypsis), and their selectivity for sunlit patches should gradually increase as Tshade decreases below Tlc.
Treecreepers were selective in their use of sun-shade patches. This result holded at different spatial scales. At the between-plots scale (habitat preferences across different forest tracts), the abundance of treecreepers was positively related to the availability of sunlit trunks when holding for the effects of tree density, prey availability, and altitude. At the within-plot scale (selection of foraging patches on trunks and thick branches), Ivlev’s electivity for sunlit patches decreased linearly as Tshade increased. Birds preferred to forage on sun exposed surfaces (electivity > 0) when Tshade was lower than » 4 ºC, but they tended to forage on shaded surfaces (electivity < 0) when Tshade was higher than » 9 ºC. The selection of sunlit trunk patches at low temperatures was not a byproduct of their higher food availabilty, because numbers of prey were much less predictable than temperature as a function of trunk exposure (sun vs. shade), the negative relationship between electivity for sunlit patches and Tshade significant after removing the effects of prey availability, and the pecking rates of focal birds did not differ between sunlit and shaded patches. Thus, the selection of sunlit patches at low temperatures can be interpreted as a behavioral thermoregulation strategy allowing birds to save energy. However, the results obtained (i.e., Ivlev’s electivity for sunlit patches became negative at temperatures well below Tlc, and no sunlit patches were used when Tshade » Tlc) lead to the rejection of the "only thermal benefits" hypothesis.
Photometric measurements of treecreeper taxidermic mounts realistically positioned on trunk surfaces, and detection times by simulated (human) predators, suggest that treecreepers were more detectable under direct solar radiation than in deep shade. Crypsis diminished in sunlit patches because of a higher image contrast, and an increased difference in perceivable coloration between bird and background, relative to shaded patches. Average detection times were significantly lower for sunlit mounts. Moreover, focal birds scanned more frequently in sunlit than in shaded patches. Thus, the observed temperature dependent variation in the selection of sunlit substrata is consistent with the "trade-off with predation risk hypothesis" predicting that prey should avoid patches where they are more detectable to potential predators. We interpret the distribution and behavior of treecreepers as indicative of a trade off between the energy savings (due to higher operative temperature and reduced metabolic costs) and the increased risk of predation (due to higher visibility and diminished crypsis) afforded by sunlit foraging patches.
POLO, V.; CARRASCAL, L.M. 1999. Shaping the body size distribution of passeriformes: habitat use and body size are evolutionarily and ecologically related. Journal of Animal Ecology 68:324-337.
The effect of habitat structure on the distribution of the number of species by body size classes has been analysed with the Passeriforms of the Western Palearctic.
Evolutionary history of the group accounted for 68% of the interspecific variation in body mass. The phylogenetic effect is highly significant from the most recent evolutive radiations (i.e. genera) towards more ancient radiations (i.e. parvorders). In a more fine-grained study with a subset of 55 passerine species living in Central Spain, phylogeny accounted for significant proportions of the interspecific variation: 62% in body mass, 27% in habitat use (foraging on ground, vs foraging in foliage) and 12% in complexity of preferred habitats.
Throughout the evolutionary history there has been a considerable concentration of species around 10-40 g (increase in kurtosis), and there have appeared species with greater body masses (increase in skewness). The higher skewness and kurtosis of the distribution of the phylogenetic component (mainly a cladogenetic one in this study) supports the role of cladogenetic processes in body size evolution.
Removing the effect of evolutionary history on present-day variation in body mass (specific component of the phylogenetic autoregressive method), the distributions of body mass of open country and woodland species are markedly different: species from woodland habitats are lighter (mainly due to the large frequency of small-sized species) and their body masses are less concentrated than in species from open country habitats. Results for the phylogenetic component are similar to those of the specific component.
Habitat use was strongly correlated with body mass in a subset of 55 species living in Central Spain: species foraging on ground being heavier than those foraging in foliage and tree branches. This result was significant working with the specific and phylogenetic components. Habitat use and complexity of preferred habitats were significantly correlated using both the specific and the phylogenetic components: species that mainly forage on ground are mainly open country species, while species that forage in pliable and slender substrata mainly inhabit woodland habitats. Complexity of preferred habitats was negatively related to body mass, although this correlation was only significant using phylogenetic residuals (specific component).
These results show that the evolutionary history of Western Palearctic Passeriforms has not produced neutral variation in body mass with respect to habitat preferences and habitat use, and supports the view that macro and micro processes have not been decoupled in the evolution of body size.
CARRASCAL, L.M.; SENAR, J.C.; MOZETICH, I.; URIBE, F.; DOMENECH, J. 1998. Interaction between environmental stress, body condition, nutritional status and dominance in mediterranean great tits (Parus major) during winter. Auk 115:727-738.
Body condition and feather growth rate of Great Tits (Parus major) were studied in relation to dominance in two contrasting Mediterranean localites during late autumn and early winter. The two localities differed in altitude, ambient temperatures (100 vs 1500 m a.s.l., and 11.7 oC vs 4.6 oC, respectively) and arthropod availability. The two study areas were similarly food supplemented (husked peanuts) throughout the study period.
Percentage of time spent at feeders was higher at El Ventorrillo (the locality with colder climate and less natural food availability), and was associated with dominance only in this locality. Number of aggressive displacements per hour suffered by each individual was higher (150-fold greater) in the area with less arthropod availability and lower temperatures. Protein reserves measured as pectoralis muscle thickness was higher at El Ventorrillo, and was positively and consistently related to dominance in both localities. Growth rate of induced feathers was slower in the locality that was colder and had less natural food availability (El Ventorrillo), but was not clearly and directly related to dominance in both localities. Only dominant adult males in El Ventorrillo could compensate the higher environmental harshness of this locality attaining a higher feather growth rate than the other sex-age classes. Feather mass asymmetry during autumn was not associated with body condition, did not change between localities, and was inversely and consistently related to dominance in both localities. The covariation pattern among variables describing bird size, access to supplemented food, body condition, feather growth rate and asymmetry was different in both localities. Only in the locality with colder temperatures and lower arthropod food availability (El Ventorrillo) larger, more dominant, Great Tits spent more time foraging on feeders, had a thicker pectoralis muscle (i.e. body condition), and grew the induced feathers at a higher rate.
MORENO, E.; BARBOSA, A.; CARRASCAL, L.M. 1997. Should congruence between intra- and interspecific ecomorphological relationships be expected? A case study with the Great Tit Parus major. Proceedings of The Royal Society London: B 264:533-539.
We studied the relationships between leg morphology and feeding posture while feeding in a population of Great Tit (Parus major) under controlled conditions to investigate to what extent morphology and ecology are linked at the individual level. From predictions generated at the interspecific level within the genus Parus (Moreno & Carrascal 1993, Ecology 74:2037-2044), we test whether interspecific and intraspecific ecomorphological relationships are consistent.
The slopes of the regressions of the leg length segments are significantly lower than those expected under isometry (geometric similarity). For IMI distance (distance from the head of the fibula to the insertion point of the iliofibularis muscle) and IMT distance (distance from the head of the tibiotarsus to the insertion point of the tibialis cranialis muscle) the regression slopes are not higher than those predicted by isometry (for IMI distance the slope is even significantly lower than that expected; p<0.001). The regression slopes of the muscular forces of M. iliofibularis and M. tibialis cranialis do not significantly differ from those expected under isometry. Therefore, only for leg length segments deviations from an isometric relationship with body mass are big enough to allow the emergence of ecomorphological patterns. Combining the coefficients of deviation from the geometric similarity there is no sufficient morphological variation between individuals to promote clear associations between morphology and hanging performance (apart from that expected by geometric similarity due to interindividual differences in body size). Thus, we should not expect significant relationships between leg morphology and average time spent hanging.
Within our population, neither leg bone lengths nor leg muscle morphology were related to the feeding posture of individuals. However, differences in body weight were correlated with inter-individual differences in time spent hanging. These results demonstrate that the association between intraspecific and interspecific ecomorphological relationships is not uniform. We argue that at the intraspecific level body weight overrides the significance of other traits that have a functional meaning at interspecific level (i.e., leg segment lengths, muscular morphology), due to isometric variation of morphological traits (muscular and squeletal) with body mass. Thus, the discrepancy between the ecomorphological associations at interspecific and intraspecific levels is the result of a problem of scale (morphological changes in evolutionary time and isometric variation of morphological traits with body mass in ecological time).
BELLIURE, J.; CARRASCAL, L.M.; DÍAZ, J.A. 1996. Covariation of thermal biology and foraging mode in two mediterranean lacertid lizards. Ecology 77:1163-1173.
Body temperatures, heat exchange rates, behavioral thermoregulation, and movement behavior (as an index of foraging mode) were studied in two widely distributed, medium sized lacertid lizards (Acanthodactylus erythrurus and Psammodromus algirus). Psammodromus algirus mainly inhabits broad-leaved forests, while A. erythrurus prefers open sandy areas with sparsely distributed vegetation. These habitat preferences parallel differences between the areas in which both genera presumably originated: Eurosaharian xeric steppes with high operative temperatures (Te’s) for Acanthodactylus, and Mediterranean open forests with lower Te’s for Psammodromus.
Field observations showed that percentage of time spent basking and basking rate (no. of basks per minute) were negatively related to Te, although average bask duration was not. Percentage of time spent moving, moving rate (no. of moves per minute), and the average duration of individual moves were inversely related to Te, and were higher in P. algirus. The percentage of total locomotion time that was spent moving in the shade was also higher in P. algirus. Behavioral thermoregulation strategies differed between both species in a laboratory thermogradient where P. algirus basked more often and for shorter periods, and selected warmer patches, than A. erythrurus. Selected body temperatures (Tb’s) in a laboratory thermogradient were significantly higher in A. erythrurus than in P. algirus. Shade Seeking Tb was higher in A. erythrurus, while Resume Basking Tb did not differ significantly between the two species. Heating and cooling rates were also different in the two species: A. erythrurus warmed more slowly, and cooled faster, than P. algirus.
Our data support the existence of a complex syndrome which combines aspects of the behavior, physiology, and ecology of both species, so that the thermal consequences of inhabiting a certain type of habitat can be counterbalanced by behavioral and physiological means that, in turn, affect movement (and hence foraging) behavior. Thus, the more active species (P. algirus) heated faster, cooled more slowly, and basked more often but for shorter periods and at warmer patches than the less active one (A. erythrurus).
CARRASCAL, L.M.; MORENO, E.; VALIDO, A. 1994. Morphological evolution and changes in foraging behaviour of island and mainland populations of Blue Tit, Parus caeruleus. A test of convergence and ecomorphological hypoteses. Evolutionary Ecology 7:25-35.
We study the leg morphology and feeding postures of two subspecies of Blue Tit (Parus caeruleus; Tenerife Island and Iberian Peninsula) and Coal Tit (P. ater; Iberian Peninsula). We search for evidence supporting the hypothesis of convergent evolution in morphological and ecological traits, and we discuss the role of ecomorphological hypotheses as predictors of foraging differences at intraspecific level. To overcome the problems introduced by environmental characteristics not related to locomotion and competition, we make observations under controlled situations to manage food quality and food access.
Island Blue Tit has longer tarsometatarsus, larger foot span, and a more proximal insertion of tibialis cranialis muscle (flexor of the tarsometatarsus) than the mainland Blue Tit. These morphological differences are consistent with the more frequent use of hanging and clinging head-up postures by the Iberian Blue Tit. Several ecomorphological hypotheses obtained at interspecific level with other taxa, have proved to be of high predictive value for explaining ecological differences considering morphological evolution. Tenerife Blue Tit and Iberian Coal Tit clearly show close convergence in both feeding postures and leg structure, although some differences in morphology were found between these two species. Convergence in foraging methods between island Blue Tit and mainland Coal Tit can be explained without considering current interspecific competition as a determinant of niche space.
TELLERÍA, J.L.; CARRASCAL, L.M. 1994. Weight-density relationships between and within bird communities. Implications of niche space and vegetation structure. American Naturalist 141:1083-1092.
The population density of a species in a given area is limited by the number of individuals that the area can support, equivalent to the amount of energy available to the population divided by the energetic requirements per individual of each species. Some studies have attempted to evaluate this capacity rule by examining the inverse relationship between density (D) and body weight (W) in several animal groups using the equation D=aWb.
We develop two different analyses that compare bird communities on two distinct spatial scales. The first approach incorporates information on foraging behavior to analyze body weight-density relationships within different assemblages (according to substrate use). This is a large scale comparison that emphasizes similarities among forest bird communities in widely different forest habitats across two continents; species from different habitats and continents are assigned to the same set of foraging assemblages. The second analysis compares communities of forest birds in the same geographic region, analyzing the allometric relationship of population density for all species in the same community. The results are compared across a systematically varying environmental gradient (foliage volume).
To analyze the relationship between mean body weight of assemblages and rate of density change with bird size, we reviewed studies on bird use of space during the breeding season in five woodland communities of North America and in two of Europe. The second analysis, focussed at the community level, was performed on data on passerine density and vegetation structure of 17 habitats in northern Spain differing in foliage height diversity and volume. For each of the 17 bird communities we calculated the slope (b) of the log-log regression model of species density on bird weight. In both within- and between-community level analyses, species heavier than 350 g were excluded as they are usually censused inaccurately by methods employed to record small passerine abundance.
Mean body weight of assemblages decreased as thinness and pliability of foraging substrates increased. The relationship between density and body mass within each assemblage, as shown by slopes from allometric equations (b), gave an inverse association with thinness of foraging substrates (log-log correlation: r=0.51, n=28). Slope b was positively correlated with mean weight (weighted by density) of birds in each assemblage; i.e., the lower the body weight of birds in assemblages, the more negative the slope of the allometric regression of D on W. These results support the view that bird assemblages show varying relationships between density and species' body weight within the niche space of communities.
Analysis at the community level provided similar results. Average weight (weighted by density of each species) of bird communities decreased with increasing foliage-volume index (log-log regression: r=-0.704, p=0.002, n=17). Slopes of the allometric regressions of D on W (b) were inversely and significantly correlated with the foliage-volume index (log-log model: r=-0.761, p=0.0004, n=17), and directly associated with average weight of bird communities (log-log model: r=0.613, n=17; significance not provided due to partial dependence between b and W).
Correlation coefficients between body weight and maximum ecological density was -0.457 in the 17 habitats of northern Spain using phylogenetic independent contrasts (p=0.011; one-tailed test). This result points out that phylogenetic relatedness alone cannot explain the consistent patterns of change of b within and between communities.
The W-D relationships, as measured by b, suggest that larger species are able to gain resources in proportion to their size, whereas smaller species have more equitable resource allocations, and so show the expected D-W associations. The results provided by this paper illuminate the previously contrasting patterns obtained about resource extraction by species within communities. The confusing patterns of the D-W relationships that are so frequently observed in birds could be at least partly due to the use of bird densities obtained from habitats that, because of their different structure, provide different niche opportunities to species of different sizes. Therefore, a more autecological approach ought to be employed in the analysis of allometric relationships linking body weight to density.
MORENO, E.; CARRASCAL, L.M. 1993. Leg morphology and feeding postures in four Parus species: an experimental ecomorphological approach. Ecology 74:2037-2044.
We investigate the ecomorphology of four phylogenetically closely related species of Parus (P. major, P. caeruleus, P. cristatus, and P. ater) that cooccur in a mixed Mediterranean woodland of Central Spain. We investigate the relation of foraging modes (hanging vs. standing back up) to leg morphology (osteology and myology). To partially control for food quality, food access and escape distance to the nearest refuge, we provided wild birds with special feeders. We also include a tentative phylogenetic analysis of the morphological and ecological (feeding postures) evolution within the genus Parus.
The relative length of the leg (femur + tibiotarsus + tarsometatarsus lengths) differed significantly among tit species, Blue and Great Tits having shorter legs than Crested and Coal Tits. Significant differences were also found in tarsometatarsus length with Great and Blue Tits having shorter tarsometatarsus than that of Coal and Crested Tits. The insertion of the M. tibialis cranialis onto the cranial surface of the tarsometatarsus is proportionally more distal in the Blue Tit than in the other three Tits.
Our results demonstrate that all tit species studied are able to use hanging and standing postures. However, there exist differences in the "preferred" foraging postures among the four Parus species. The Blue Tit used hanging in a higher proportion than the other three Parus species; the Crested Tit spent the lowest proportion of time hanging. The Blue Tit and the Crested Tit also showed the most contrasting hindlimb morphological designs. The ranking of the four tit species according to hanging frequency is consistent with the predicted order derived from morphofunctional hypotheses (including muscles iliotibialis cranialis, gastrocnemius, fibularis brevis, and tibialis cranialis, relative length of legs, and body mass).
Blue Tit is the species with shortest legs. It has the main flexor muscles of the hip and intratarsal joints (M. iliotibialis cranialis and M. tibialis cranialis respectively) modified in the direction that increases the force of their action (i.e., closing the joints). M. fibularis brevis is also modified to strengthen its action maintaining the tarsometatarsus at its position once it is flexed. Pars interna of M. gastrocnemius, the main extensor of the ankle, is, on the contrary, partially atrophied in the Blue Tit. This morphological pattern fits the mechanical requirements for hanging; i.e., morphological design should tend to counteract the force of gravity (the main force acting upon the bird while hanging) which pulls the body downwards and tends to open the leg joints. Conversely, the Crested Tit has the longest legs. The main extensor muscle of the ankle, M. gastrocnemius, is modified by the addition of extra fibers to its pars interna, increasing ankle extension, as the force exerted by a muscle is related to its number of fibres. Leg flexor muscles are not as well developed as in the Blue Tit (decreased leg flexion power). This hind limb morphology better fits the mechanical requirements for standing; i.e., morphological design should tend to counteract the force of gravity, which, while standing, tend to close the leg joints. It is plausible to hypothesize an unspecialized postural selection as the primitive behavioural character state for Parus.
CARRASCAL, L.M.; TELLERÍA, J.L.; VALIDO, A. 1992. Habitat distribution of canary chaffinches among islands: competitive exclusion or species-specific habitat preferences? Journal of Biogeography 19:383-390.
The habitat distribution between-islands of Common Chaffinch (Fringilla coelebs L.) related to the presence of the potential competitor species Blue Chaffinch (Fringilla teydea Moquin-Tandon), has been studied in the Canary Islands (Tenerife and El Hierro). Common Chaffinch was significantly denser in the pine woods of El Hierro than in Tenerife, while Blue Chaffinch was only present in Tenerife. The structure of the vegetation was very different between the pine woods of the two islands. In the pine wood of El Hierro Common Chaffinch selected the places more covered by grasses and foraged mainly in the foliage.
The habitat selection pattern observed in Hierro was congruent with that obtained for the continental subspecies at the North of the Iberian Peninsula. Therefore, an empirical model was developed to predict density variations of Common Chaffinch in continental pine woods of Northern Spain. This continental model (not subjected to the influence of the potential competitive effect of the Blue Finch) was used to predict the abundance of common chaffinches at the pine woods of El Hierro and Tenerife. The similarity between the densities predicted by the non-competitive continental model and the observed ones in the pine woods of El Hierro and Tenerife points out that the presence of Blue Chaffinch is not relevant to explain the differences in Common Chaffinch density between islands. Habitat preferences of Common Chaffinch quantitatively explains density differences observed between El Hierro and Tenerife. These results clearly show the relevance of habitat structure determining the patterns of presence and density of the Common Chaffinch between islands. Data obtained agrees with the species-specific habitat preference hypothesis, being the competitive exclusion hypothesis not justifiable, at least in ecological time.
CARRASCAL, L.M.; TELLERÍA, J.L. 1991. Bird size and density: a regional approach. American Naturalist 138:777-784.
Within groups of taxonomically-related, ecologically similar organisms, large individuals demand more from environmental resource pools than small ones, but also tend to occur at lower densities. Some studies have proved the inverse relationship between weight (W) and density (d) in several zoological groups. This relationship is well fitted to the equation d=aWb (log(d) = log(a)+b·log(W)), where d and W are normally obtained from bibliographical reviews. This relationship is unsatisfactory in the case of birds apart from raptors, although there has been no convincing explanation of this result. We review several aspects responsible of this lack of association.
Densities may be classed as regional densities (dreg) when referring to density reached by species in a region which includes diverse types of habitats, and ecological densities (di) when referring to a defined habitat (common approach in studies on communities of small vertebrates). The maximum ecological density (dmax) places each species at its most optimum habitat in the area. It may be obtained from an analysis of di distribution in the study area. According to animal distribution models, these habitats should be the first to be saturated when the regional population level increases. This situation favours size-dependent restrictions to animal abundance. The use of dmax attenuates the distorting effect of habitat preferences. The correlation between dmax and W will thus be better than that obtained using a random selection of ecological densities (di) of each species in any of the regions' habitats, as it is usually done. On the other hand, given that dreg attenuates the potential distorting effect of the high variability of random selected di values by weighing the regional significance of each habitat in species abundance, it will foreseeable correlate better with W.
In this paper we investigate the relationship between d and W in small insectivorous birds of the Basque Country (North of Spain). Ecological densities (di) were estimated using line transects in the 10 most extensive habitats of the region. While this method does not completely reduce the effects of differences in species detectability, it is considered adequate for an approximate characterization of small passerines and related species densities. Attention was thus centered on 47 species weighing between 6 (Regulus ignicapillus) and 160 g (Picus viridis).
The relationship between di and W for all species and habitats showed a "polygon" of points in which each interval of W (similar sized species) provided a high variability of values of di. The upper profile of this polygon descended as W increased, corresponding to the maximum densities (dmax) of each species, while the lower profile was parallel to the axis of X (lower densities). The mean result from the correlations among random selected di for each species (null model) in this "polygon" show that r=-0.217 with 19 mean degrees of freedom. The confidence interval is from -0.332 to -0.102 at 1% and -0.305 to -0.129 at 5%.
Using the regression analysis of W on dmean, dmax and dreg for the 47 species we obtained that the correlations were negative and the value of r exceeded that from the null model. The use of the dmax thus seems to improve the relationships between d and W (b=-0.62; explained variation: 25.14%). The results of this study demonstrate a negative relationship between d and W in small insectivorous birds, and that it is important to consider the variability of densities due to the uneven distribution of the species by habitats. The similarity between the absolute values of our coefficients b and those derived from energetic allometric equations (b=0.75) seems to point out to the idea that the amount of energy used by these small insectivorous birds is independent of their body size.
DÍAZ, J.A.; CARRASCAL, L.M. 1991. Regional distribution of a Mediterranean lizard: influence of habitat cues and prey abundance. Journal of Biogeography 18:291-297.
We analyzed the local variations in the relative abundance of a common Mediterranean lizard species, Psammodromus algirus (L.) 1758 in 19 habitats of Central Spain. We considered the effects of habitat structure, flora, prey abundance, climate and potential competition with other Lacertidae. Our results were interpreted in the light of explanatory hypotheses that can probably be applied to this and other insectivorous lizards at a broader geographycal scale.
The lack of significant correlation between the abundance of P. algirus and the scores on the climatic gradient (CPC) indicates that mesoclimate is not truly important for the distribution and abundance of this lizard species. The presence of other lacertid species seemed to be unrelated with the distribution and abundance of P. algirus at a regional scale. The effects of plant taxonomy on the abundance of P. algirus vanished with the exclussion of other environmental attributes that appeared to be more important for the biology of the species. This result showed that floristic gradients covariating with the abundance of P. algirus did not have true causal effects but merely reflected underlying biological processes.
Nevertheless, a single structural component (the development of the low shrub canopy) accounted for more than two thirds of the variance observed in the abundance of P. algirus. The underlying bases for this relationship are two basic requirements of lizard biology: thermoregulation and predator avoidance. Low shrub cover above 20 cm in height generates a continuous sun/shade gradient along which thermoregulating animals would be able to select their position, and the same low, dense bushes would offer a good refuge for flight no matter their taxonomical identity. This would ultimately enhance survival (by favouring optimal body temperatures and reducing predation pressure) and hence produce more numerous populations.
Despite the high summer abundance of arthropod prey in Mediterranean environments, their availability influenced the abundance of P.algirus along a gradient of habitats over the main effects attributable to habitat structure, since the partial correlation obtained was highly significant (r=0.667, p=0.0025). The intriguing effect of food abundance on lizard population levels could be interpreted as the consequence of a trade-off between the conflicting demands of getting enough food and avoiding the numerous predators that feed on P. algirus.
The habitat selection patterns obtained were validated by means of jacknife and randomization procedures. Predicted and observed values of lizard density were significantly and highly correlated.
CARRASCAL, L.M.; POLO, V. 1999. Coal tits, Parus ater, lose weight in response to chases by predators. Animal Behaviour 58:281-285.
Theoretical models predict that birds should decrease their body mass in response to increased predation risk because lighter birds take off faster and are more manoeuvrable. We studied the effect of predation risk by chasing coal tits in large outdoor aviaries thus simulating an attempt to capture them. With this increase in predation risk, both perceived and actual, coal tits lost significantly more weight than in a control situation when they were not pursued. This pattern was attributable to a smaller gain in weight only during the day; nocturnal weight did not change in relation to diurnal predation risk. The lower daily weight gain was not consistent with predictions from models of interrupted foraging, but was consistent with predictions from risk adjustment models. Moreover, there was no difference in weight gain over 2-h periods that included a 1-h fast and those in which feeding was ad libitum, suggesting that coal tits could easily regain their body mass after a predator had interrupted their feeding. Our results therefore suggest that pursuit by predators leads to a decrease in the body mass of small birds.
Maximizing the average rate of energy intake (profitability) may not always be the optimal foraging strategy for ectotherms with relatively low energy requirements. To test this hypothesis, we studied the feeding behaviour of captive insectivorous lizards Psammodromus algirus, and we obtained experimental estimates of prey mass, handling time, profitability, and attack distance for several types of prey.
Handling time increased linearly with prey mass and differed significantly among prey types when prey size differences were controlled for, and mean profitabilities differed among prey taxa, but profitability was independent of prey size. The attack distance increased with prey length and with the mobility of prey, but it was unrelated to profitability. Thus, lizards did not seem to take account of the rate of energy intake per second as a proximate cue eliciting predatory behavior.
This information was combined with pitfall-trap censuses of prey (in late April, mid-June and late July) that allowed us to compare the mass of the prey captured in the environment with that of the arthropods found in the stomachs of sacrified free-living lizards. In April, when food abundance was low and lizards were reproducing, profitability had a pronounced effect on size selection and lizards selected prey larger than average from all taxa except the least profitable ones. As the active season progressed, and with a higher availability of food, the number of prey per stomach decreased and their mean size increased. The effect of profitability on size selection decreased (June) and eventually vanished (July-August). This variation is probably related to seasonal changes in the ecology of lizards; e.g. time minimization in the breeding season as a means of saving time for nonforaging activities versus movement minimization by selecting fewer (but larger) prey in the postbreeding season. Thus, the hypothesis that maximizing profitability could be just an optimal strategy for a terrestrial ectothermic vertebrate was supported by our data.
Carrascal, L.M.; Moreno, E.; Mozetich, I. 1995. Ecological plasticity of morphological designs. An experimental analysis with tit species. Canadian Journal of Zoology 73:2005-2009.
To determine whether behavioural plasticity in foraging modes is contingent upon the phenotype of the species, we analyze the frequency of use of foraging postures (standing vs hanging) under the selection pressure of predation risk, of two contrasting tit species in leg morphology (Blue Tit, Parus caeruleus, and Crested Tit, P. cristatus). Results demonstrated that leg morphology of the Blue Tit enables it with greater ecological flexibility in foraging postures than that of the Crested Tit. The constrained foraging posture was to hang. Observed interspecific differences in ecological flexibility are not attributable to interspecific differences in morphological variability within species. Morpho-functional bases of ecological flexibility in foraging postures are discussed. Morphological design may be considered as an important factor in explaining niche width and potential for niche shifts, as morphology might determine the range of ecological "space" within which a species is allowed to move.
Carrascal, L.M.; Moreno, E.; Tellería, J.L. 1990. Ecomorphological relationships in a group of insectivorous birds of temperate forests in winter. Holarctic Ecology 13:105-111.
We examined the relationships between morphology and foraging behaviour in a group of insectivorous birds wintering in temperate mixed forests in northern Iberia. Using principal components analysis we reduced 11 biometric variables to three major morphological components and 20 foraging categories to four major ecological factors. The relative length of the tarsometatarsus and bill morphology were the most important morphological variables predicting foraging ecology. Birds exploiting distal parts of trees and foliage were generally smaller and had relatively longer tarsometatarsi than those foraging on trunks. Foraging on the ground and branches of medium diameter was associated with bill thickness. Ecomorphological patterns were discernable at the level of substrate use and foraging methods, but bear no relation to selection of tree species or foraging height. Morphology correctly predicted niche breadth and interspecific overlap. In Parus spp. interspecific differences in bill shape could explain 63% of the interspecific segregation according to substrate use (R2=0.63, p<0.01).
Carrascal, L.M.; Moreno, E. 1992. Proximal costs and benefits of heterospecific social foraging in Great Tit Parus major. Canadian Journal of Zoology 70:1947-1952.
The feeding and vigilance schedules of Great Tits (Parus major) at artificial feeders were studied in winter. We compared the behaviour of birds that foraged in pars ( mono- or hetero-specific) and solitarily. The percentage of time spent foraging was higher when a Great Tit was with an individual belonging to a subordinate species rather than with another Great Tit or a member of the dominant species. When a bird was solitary, time spent foraging was less than when it was with an individual belonging to a subordinate species, and greater than when it was with one belonging to the dominant species. The proportion of time spent vigilant did not differ between Great Tits that foraged with an individual from a dominant species or a subordinate species or with a conspecific, but was higher for solitary birds. Our results indicate that not all species gain advantages by foraging in heterospecific dyads. Individual Great Tits benefited from the presence of Coal, Blue, and Crested tits, both by a lengthening of the time spent foraging at feeders, and by conversion of vigilance time into feeding time. Dominant Great Tits seek the presence of subordinate Parus species to enhance foraging efficiency under safer conditions (using other species as early warners), the converse being true for subordinate species (i.e., they should avoid dominant Great Tits and terminate heterospecific social foraging more often).
Vigilance time of solitary Great Tits, Nuthatches, and three other Parus species was not negatively correlated with interspecific dominance status. In these small passerines, vigilance does not seem to be directed towards the detection of a competitor species. Surveillance for predators and detection of competitor species may be synchronous, nonexclusive tasks. Scanning rate (number of scans per minute of feeding) was the same for Great Tits that fed either solitarily or in pairs, but mean scan duration was significantly shorter for individuals in groups than when solitary. Individual Great Tits benefited from the presence of Coal, Blue, and Crested tits (subordinate Parus species), by a lengthening of the time spent at feeding patches and by a conversion of vigilance time into feeding time.
CARRASCAL, L.M.; LÓPEZ, P.; MARTÍN, J.; SALVADOR, A. 1992. Basking and antipredator behaviour in a high altitude lizard: implications of heat-exchange rate. Ethology 92:143-154.
This paper presents an observational and experimental study of the basking behaviour and heat exchange rate of the montane lizard Lacerta monticola. The results obtained by these procedures were coupled in order to understand behavioural mechanisms promoting effective thermoregulation at high altitudes. Heating rate was higher when body size was smaller, and substrate temperature and sunrays incidence angle were higher. The lizards cooled faster when body size and substrate temperature were lower, and when the body temperature of the lizard going into shadow was higher. Time exposed to sun and mean duration of basking periods were longer early in the morning, while bask frequency increased through the morning. Our results suggest that time devoted to basking is mainly obtained by regulating bask duration. Lizards obtained the necessary time for heating by means of long basking periods. Mean travel distance per minute and distance to the nearest refuge increased from early morning to midday. These behavioural variables were tightly correlated with the expected heating rate of individuals. Body size affects thermoregulatory behaviour as well as locomotor activity. Juvenile lizards, with small body mass and high surface-to-volume ratios, were subjected to faster heating and cooling rates, basked more frequently than adults (but during shorter periods), and devoted more time to locomotion than adults. The thermoregulatory behaviour of L. monticola is the result of the combination of shuttling heliothermy by basking and the exploitation of thermal opportunities offered by patches in shade through thermal exchange with the substrate.
Carrascal, L.M.; Bautista, L.M.; Lázaro, E. 1993. Geographical variation in the density of the White Stork Ciconia ciconia in Spain: influence of habitat structure and climate. Biological Conservation 65:83-87.
The spatial variation in the density of white storks Ciconia ciconia in Spain is analysed with respect to landscape and meteorological variables. The density of breeding pairs in 1985 was negatively correlated with surface cover of woodlands and shrublands, and positively correlated with the area of dry or wet grasslands, reflecting .food availability and foraging preferences of the storks. Average minimum temperature in April-May (the first .few days after hatching) was also negatively correlated with stork density, and the reproductive success in a colony at El Tietar (Avila) was inversely correlated with the number of days with precipitation in May. The negative influence of minimum temperature and precipitation on breeding density appears to be linked with the mortality of recently born nestlings. Practical recommendations are made for the conservation of the Spanish population of white storks through incentive use of pastures, meadows and 'dehesas' for cattle grazing. Reintroduction efforts must be direct towards zones having large areas of these habitats, and mild weather.