Factors affecting population regulation of a colonial vulture I ~ NIGO ZUBEROGOITIA, 1 * JOS  E ENRIQUE MART  INEZ, 2 JOS  E ANTONIO GONZ  ALEZ-OREJA, 3 JUAN MANUEL P  EREZ DE ANA 4 & JABI ZABALA 5 1 Estudios Medioambientales Icarus, S.L. C/ San Vicente. 8. 6 ª Planta. Dpto 8. Edificio Albia I. 48001, Bilbao, Bizkaia, Spain 2 Bonelli 0 s Eagle Study and Conservation Group, Apdo. 4009, 30080 Murcia, Spain 3 Escuela de Biolog  ıa, BUAP, Benem erita Universidad Aut  onoma de Puebla, Ciudad Universitaria, Edificio 112-A, 72570 Puebla, Mexico 4 Departamento Sostenibilidad y Medio Natural, Diputaci  on Foral de Bizkaia. Avda Lehendakari Aguirre, 9-2. 48014 Bilbao, Spain 5 Sebero Otxoa 45 5 B, 48480 Arrigorriaga, Bizkaia, Spain 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 depen- dence 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 neg- ative effect of rainfall. Keywords: density dependence, eyrie quality, Griffon Vulture, Gyps fulvus, population regulation, productivity. Where habitats are not disturbed, most bird popu- lations remain relatively stable in abundance over long periods (Newton 2013). Many factors influ- ence population growth, but only density-depen- dent factors, where effects increase with crowding, can bring the size of a population under control (Rockwood 2015). In territorial species, two hypotheses have been proposed to link population regulation to density-dependent changes in demo- graphic parameters (Fretwell & Lucas 1970). Where habitat quality differs between territories, the habitat heterogeneity hypothesis (HHH) states that, as population density rises, an increasing *Corresponding author. Email: zuberogoitia@icarus.es © 2018 British Ornithologists’ Union Ibis (2019), 161, 878–889 doi: 10.1111/ibi.12687