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

Abstract

Methods to evaluate population trends have recently received particular attention because of perceived declines in several species during the 20th century. We investigated whether age at first breeding could be used as an ''early warning signal'' to detect possible changes in population trends in long-lived species with deferred maturity using data from the Spanish imperial eagle (Aquila adalberti) population in Donana National Park (Spain). This bird of prey is an endangered species that has suffered a rapid decline in this population during the last 10 years. As a result of our 27-year monitoring ) study, we detected that an increase in immature breeding birds occurred before population decline became evident. The proportion of immature-plumaged breeders in the population was significantly higher during the period of decline than during the period of stability. In our case, more than 10% of immature breeders can be considered as an ''early warning signal'' that anticipates population decline. Owing to the ignorance of this warning signal, urgent actions for the recovery of this eagle population started 10 years later than necessary, and when population size had been reduced.

Floater strategies and dynamics in birds, and their importance in conservation biology: towards an understanding of nonbreeders in avian populations

“…This underscores the importance of monitoring not just the numbers and locations of breeders, but also their ages. This is an essential concept in conservation biology: monitoring the composition of recruits from the floating population may allow early prediction of a future population decline at a time when it might still be possible to prevent the decline (Kenward et al, 2000;Ferrer et al, 2003). Although regular monitoring of the age structure of territorial pairs may not be feasible in species for which age cannot easily be established, this is a potentially reliable method for forecasting changes in population viability in many other species, and provides a useful tool for improved conservation biology.…”

Markov models of territory occupancy: implications for the management and conservation of competing species

“…Schwartz et al 2002;Ferrer et al 2003;Naves et al 2003;Ontiveros et al 2004;WhitWeld et al 2004;Pounds et al 2006). In spatially structured populations, the study of the conditions for species coexistence is relevant for conservation and management, as interactions with a superior competitor may lead to the competitive exclusion of competing species (Hanski 1983;Nee and May 1992;Tilman 1994;Moilanen and Hanski 1995;Palomares et al 1996;Sergio et al 2003;Carrete et al 2005).…”

Government-sponsored Falconry Practices, Rodenticides, and Land Development Jeopardize Golden Eagles (Aquila chrysaetos) in Western China

“…Future studies should use data from marked individuals and probabilistic sampling to investigate other demographic variables, such as survival of nestlings from harvested and nonharvested nesting territories on the Navajo Nation, effects of harvest on age structure of the regional population, and density of nesting pairs and individuals. Although our sampling methods did not allow us to confirm ages for the majority of territory occupants, aging of nesting eagles should be a part of future monitoring efforts on the Navajo Nation because increasing rates of breeding subadults would indicate a declining floater pool (Hunt 1998, Ferrer et al 2003, Penteriani et al 2006. We suggest future studies of long-lived raptors on the Navajo Nation and elsewhere use design-based sampling, survey methods and models to account for imperfect detection, and incorporate environmental factors known to affect demographic rates of raptor populations.…”