title: Spatial heterogeneity in resource distribution promotes facultative sociality in two trans-Saharan migratory birds.
abstract: BACKGROUND
  Migrant populations must cope not only with environmental changes in different biomes, but also with the continuous constraints imposed by human-induced changes through landscape transformation and resource patchiness. Theoretical studies suggest that changes in food distribution can promote changes in the social arrangement of individuals without apparent adaptive value. Empirical research on this subject has only been performed at reduced geographical scales and/or for single species. However, the relative contribution of food patchiness and predictability, both in space and time, to abundance and sociality can vary among species, depending on their degree of flexibility.
  
  
  RESULTS
  By means of constrained zero-inflated Generalized Additive Models we analysed the spatial distribution of two trans-Saharan avian scavengers that breed (Europe) and winter (Africa) sympatrically, in relation to food availability. In the summering grounds, the probability of finding large numbers of both species increases close to predictable feeding sources, whereas in the wintering grounds, where food resources are widespread, we did not find such aggregation patterns, except for the black kite, which aggregated at desert locust outbreaks. The comparison of diets in both species through stable isotopes revealed that their diets overlapped during summering, but not during wintering.
  
  
  CONCLUSIONS
  Our results suggest that bird sociality at feeding grounds is closely linked to the pattern of spatial distribution and predictability of trophic resources, which are ultimately induced by human activities. Migrant species can show adaptive foraging strategies to face changing distribution of food availability in both wintering and summering quarters. Understanding these effects is a key aspect for predicting the fitness costs and population consequences of habitat transformations on the viability of endangered migratory species.
authors: Department of Conservation Biology, Estación Biológica de Doñana, Consejo Superior de Investigaciones Científicas, Sevilla, Spain. ainara@ebd.csic.es
authors: Ainara
authors: A
authors: Cortés-Avizanda
authors: Pablo
authors: P
authors: Almaraz
authors: Martina
authors: M
authors: Carrete
authors: José A
authors: JA
authors: Sánchez-Zapata
authors: Antonio
authors: A
authors: Delgado
authors: Fernando
authors: F
authors: Hiraldo
authors: José A
authors: JA
authors: Donázar
mesh_terms: D000352:Africa, Northern; D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D044822:Biodiversity; D001717:Birds / Q000502:physiology*; D002247:Carbon Isotopes / Q000378:metabolism; D002417:Cattle; D005247:Feeding Behavior; D005502:Food; D005843:Geography; D006801:Humans; D007553:Isotope Labeling; D008954:Models, Biological; D009587:Nitrogen Isotopes / Q000378:metabolism; D012621:Seasons; D012919:Social Behavior; D013045:Species Specificity
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes