|Conservation Biology Group|
New information on survival of Bonelli’s Eagle
Two new studies conducted by the Conservation Biology Group and French researchers reveal new information on survival of Bonelli’s Eagle populations in Catalonia and France.
Most methods of survival estimation require substantial human and economic investment in the long term, particularly in species occurring in low densities, the case of most endangered species. An alternative to traditional recapture (CR) methods is estimation of adult survival based indirectly on either age ratios (AGR) or turnover rates (TOR) in territorial species. These 2 methods are applicable to bird species in which recruited individuals enter into the breeding population whilst still exhibiting the external traits that distinguish those animals from experienced adults. The main advantages of these methods are that survival can be easily estimated for all monitored individuals after just 1 or 2 breeding seasons and that disturbance to the species is minimized. In this sense, a new study published in the Journal of Wildlife Management uses data from a long-term monitoring survey of 2 populations of the endangered Bonelli's Eagle, one in Catalonia and the other in Provence and Languedoc-Roussillon. We evaluated survival estimates using the AGR and TOR methods and compared them with CR methods and provide suitable corrections for refining survival estimates based on indirect methods.
Results revealed that survival was higher in females than in males, and that individuals from the French population survive better (0.87-0.88, depending on the method, during 1999-2008) than individuals from the Catalan population (0.84-0,86, depending on the method, during 2002-2008).
The article concludes that indirect methods, which should not be regarded as a substitute of CR methods, will allow wildlife managers and researchers to estimate accurately adult survival in a territorial species over a short period of time and to monitor survival across populations over large geographic ranges and over time.
In addition, a second study published in the scientific journal Ibis evaluated the effects of age, territoriality and reproduction on survival in Bonelli’s Eagle through multistate capture-mark-recapture analyses on a long-term dataset. Monitoring was carried out in southeast France (1990–2008) and involved the surveying of territorial Eagles, the marking of fledged chicks, and the recording of resightings and recoveries of marked non-territorial and territorial birds.
Results showed that survival improved with age; yearly survival was estimated at 0.479 for fledglings (to 1 year of age), 0.570 for 1- and 2-year-olds, and 0.870 for 3-year-old and older individuals. Finally, 4-year-old and older successful breeders were more likely to breed the following year than failed adult breeders (0.869 vs. 0.582), suggesting that the cost of reproduction is small in comparison with the variation in quality among individuals or their territories.
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