The proportion of immature breeders as a reliable early warning signal of population decline: evidence from the Spanish Imperial eagle in Doñana

Data from the Spanish imperial eagle Aquila adalberti population in Donana National Park, southwestern Spain, were analyzed by linear regression and one-way analysis of variance to evaluate the relationships between percent of immature breeding birds and population saturation levels, mortality, and number of pairs. The imperial eagle is the most endangered bird of prey in Europe. The data, which covered the period 1976 2002, indicated a stable population through 1992 and a serious decline thereafter. The percentage of immature breeders differed significantly between the periods 1976 1991 and 1992 2002, increasing in the latter period as the population size decreased. A significant relationship was noted between adult eagles found dead and percent of immature breeders.