Rescue of a small declining population of Spanish imperial eagles

Abstract

The Spanish imperial eagle (Aquila adalberti) is one of the most endangered raptors in the world. The population of Doñana National Park (south-western Spain) suffered a dramatic decline from 1990 on, after a long period of stability. The high adult mortality due to poisoning was the main cause of the decline, decreasing fecundity and biasing the offspring sex ratio to males due to a higher proportion of non-adult breeders in the population. In face of the imminent extinction, an urgent multi-action conservation plan was implemented in 2004. Supplementary feeding throughout the year with live wild rabbits was undertaken to prevent breeders from foraging outside the National Park and therefore reducing adult mortality by poisoning. Likewise, the population was reinforced with the release of young eagles (mainly females) by hacking techniques. After implementing the plan, the annual adult mortality decreased from 12% in the declining period to 2.1% during the recovery period. The lower adult mortality resulted in a lower population turnover and thus in an increasing average age of breeders (proportion of non-adult plumage decreased from 21.3% to 8.9%). Accordingly, the fecundity recovered to values close to those prior to decline (from 0.6 to 1.3young/pair) and the sex ratio was balanced again (from 81% to 48% males). In fact, the predicted population persistence increased up to nearly six times after the application of the action plan. Therefore, the conservation actions applied were effective in a relatively short period and made possible the rescue of the threatened population from the extinction vortex.