The population size of the Pyrenean population of the bearded vulture Gypaetus barbatus, the most important viable wild population in Europe, remains poorly known, despite its status as endangered in Europe. In this study, the abundance of immature vultures and adults (both territorial and floaters) was estimated in the Aragón region, the core area of the Pyrenean population, to provide an indicator of the total Pyrenean population. Using artificial feeding stations specifically designed for bearded vultures as survey points, 25 censuses were conducted during March 2010 in which 873 sightings (106 of individually marked birds and 767 of unmarked birds) were recorded. By applying the mark-recapture method, the total population of bearded vultures in the study area during the winter period was estimated to be 328 individuals (95% CI: 279-391 individuals). Separate estimates by age groups suggested a population of 129 immature individuals (95% CI: 107-160) and 227 adults (95% CI: 173-311). The adult floating population was estimated at 49 individuals and the adult nonbreeding fraction was 93 individuals. Compared to previous censuses, the results suggest that although the reproductive fraction is increasing, the total population still has some problems: the immature fraction is not increasing, as predicted in previous studies, and a high proportion of adults are not contributing effectively to the breeding process. Conservation measures should focus on these two issues to increase the viability of the Pyrenean population.
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