Variations in the age of mates as an early warning signal of changes in population trends? The case of Bonelli's eagle in Andalusia

Methods to evaluate population trends have recently received particular attention because of the decreasing patterns shown by several species since the nineteenth century. We have studied demographic traits of the Bonelli's eagle (Hieraaetus fasciatus) population of Andalusia (southern Spain) for 20 years (1980-2000). This species is endangered and is suffering a rapid decline in most of its distribution range. Although our study population has remained stable for the last 10 years, both age at first breeding and productivity have decreased during the last 5 years. We propose that age at first breeding could be used as a potential early warning signal to detect possible future changes in population trends of long-lived species with deferred maturation age. In particular, in this species spatial variation in demographic traits could help to detect whether unnatural events such as shooting are decreasing age at first breeding and productivity. If occupation of territories by non-adults is caused by human persecution, conservation efforts in the particular case of Bonelli's eagle have to be directed to protect those breeding territories where an increase in the percentage of pairs containing at least one non-adult individual would be detected.