Factors affecting population regulation of a colonial vulture

• Published in: Ibis (London, England) England), 2019-10, Vol.161 (4), p.878-889
• Main Authors: Zuberogoitia, Iñigo, Martínez, José Enrique, González‐Oreja, José Antonio, Pérez de Ana, Juan Manuel, Zabala, Jabi
• Format: Article
• Language: English
• Subjects: Animal breeding; Breeding; Breeding success; Caves; Cliffs; Colonies; Density dependence; Dependence; eyrie quality; Fecundity; Griffon Vulture; Gyps fulvus; Heterogeneity; Hypotheses; Juveniles; Ledges; Nesting; Population density; Population regulation; productivity; Rain; Rainfall; Reproduction; Reproductive failure; Survival; Weather
• ISSN: 0019-1019; 1474-919X
• DOI: 10.1111/ibi.12687

Two hypotheses have been proposed to link population regulation to density‐dependent changes in demographical parameters: the habitat heterogeneity hypothesis (HHH) states that, as population density rises, an increasing proportion of individuals are forced to occupy low‐quality territories, which provokes a decline in average per‐capita survival and/or productivity although some individuals show no decline in fecundity; and the individual adjustment hypothesis (IAH), which suggests that increased densities lead to reductions in survival and/or fecundity by enhancing agonistic interactions, which affect all individuals to a similar extent. However, density‐dependent effects can be affected by density‐independent factors (DIF), such as weather. We test the effects of density dependence on annual reproductive success in Griffon Vultures Gyps fulvus at four spatial scales, nest‐site, cliff, colony and metacolony, in northern Spain from 2008 to 2015. Our results showed most support for the HHH at all scales. At the colony and cliff scale, IAH and DIF had similar importance, whereas there was little evidence of IAH at the metacolony and the nest scale. The best protected eyries (caves, potholes and sheltered ledges) produced the most fledglings and were used preferentially, whereas low‐quality eyries (exposed ledges or open crevices) were used only when the number of breeders increased. The significant interaction between breeding failure and density found for the more exposed eyries suggests that at higher densities, breeding pairs are forced to use poorer nesting areas, and the negative effect of density at the cliff scale could be due to the combined effect of a higher proportion of pairs using low‐quality eyries and the negative effect of rainfall.