DENSITY-DEPENDENT EFFECTS ON PRODUCTIVITY IN THE GRIFFON VULTURE GYPSFULVUS - THE ROLE OF INTERFERENCE AND HABITAT HETEROGENEITY

Griffon Vultures Gyps fulvus in northern Spain were studied between 19 69 and 1994. The number of breeding pairs increased from 221 in 1969-1 975 to 1395 in 1994. The annual population growth rate decreased in th e last 5 years, and this may reflect population regulation through den sity-dependent phenomena, Breeding success was monitored in 1994 and e xamined in relation to colony size, density of breeding pairs within a radius of 25 km (regional density), climate, human disturbance and fo od availability, We also recorded whether the year of first occupation of each nest site was before 1989 or after 1989 and whether or not th e nest had a rocky shelter. The probability of successfully raising yo ung declined as the regional density increased, which suggests that re source limitation would take place at foraging sites because the Griff on Vulture scavenges socially and no permanent feeding hierarchies are established. The other significant variable was the year of occupatio n of the nest; nests occupied after 1989 had a lower probability of ra ising a chick. The increase in the regional density of Griffon Vulture s produced a decrease in the productivity at both optimal and suboptim al nest sites. This suggests that density-dependent regulation of bree ding success operates through interference and that all the individual s in a colony are similarly affected. In birds of prey, prevalence of interference or habitat heterogeneity may be dependent on the social s trategy of each species in space exploitation.