Quick methods for evaluating survival of age‐characterizable long‐lived territorial birds

Abstract

Survival is a key life‐history trait in animals. However, 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. The main constraints of indirect methods are that the assumptions are more restrictive than in CR methods, and survival estimates, although comparable between sites and years, may be biased. We used data from a long‐term monitoring survey of 2 populations of the endangered Bonelli's eagle (Aquila fasciata), one in Catalonia (NE Spain) and the other in Provence and Languedoc‐Roussillon (SE France). 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. In Catalonia (2002–2008), survival was estimated at 0.84 by CR methods (SE = 0.047; n = 25 radio tagged eagles), at 0.86 by the corrected AGR method (SE = 0.011; n = 558 bird * year), and at 0.86 by the corrected TOR method (SE = 0.022; n = 547 bird * year). In France (1999–2008), survival was estimated at 0.88 by CR methods (SE = 0.040; n = 45 darvic banded eagles), at 0.87 by the corrected AGR method (SE = 0.015; n = 443 bird * year), and at 0.87 by the corrected TOR method (SE = 0.015; n = 438 bird * year). All analyses suggest that females survive better than males and that individuals from the French population survive better than individuals from the Catalan population. We conclude 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. © 2011 The Wildlife Society.

From local monitoring to a broad‐scale viability assessment: a case study for the Bonelli's Eagle in western Europe

“…By contrast, the French and eastern Iberian populations (FRA, CAT, PVAL, and MUR) have fairly good levels of productivity, but with the lowest survival rates (Carrete et al 2005, del Moral 2006, Herna´ndez-Matı´as et al 2011a, which agrees with the view that, due to both electrocution and direct persecution (the chief causes of mortality in this species), the highest mortality risk is to be found in highly humanized areas . The populations from central Iberia (ARA and May 2013 253 BROAD-SCALE PVA FOR BONELLI'S EAGLE GUA) are more self-sustainable because they have better survival values than eastern populations, although productivity is slightly lower given the poorer local environmental conditions (Real 2004, Carrascal andSeoane 2009).…”

“…Territorial recruitment mostly occurs between three and four years of age . During the territorial phase, individuals exhibit strong pair-bonding behavior and establish home ranges of ;50 km 2 , with strong fidelity to the breeding area throughout the year and from one year to the next , Herna´ndez-Matı´as et al 2011a. Most eagles in western Europe nest on cliffs, although in southern Portugal, most pairs nest in trees.…”

“…Adult survival was estimated (except in France) from turnover rates of territorial birds based on plumage ages, but was corrected by the proportion of nonobservable replacements (NOR; i.e., the replacement of an adult-plumaged territorial eagle by another adultplumaged eagle; see details in Herna´ndez-Matı´as et al 2011a and Appendix B). We estimated the adult survival for each population as the geometric mean of annual survival probabilities.…”

“…Individuals that had dispersed from one population to another (either birds attempting to recruit or already-recruited birds) were subject to the vital rates of the destination population. Once a bird becomes territorial, no further dispersal was allowed for since empirical data suggest that such behavior is extremely rare in Bonelli's Eagle (Herna´ndez-Matı´as et al 2011a). All models accounted for demographic stochasticity in all vital rates and environmental stochasticity in adult survival and fertility, which was estimated from their observed temporal variance.…”

Effects of rearing conditions on natal dispersal processes in a long‐lived predator bird

“…Indeed, pair formation in Bonelli’s eagles mostly occurs when individuals occupy vacancies left by territorial birds who have died (Hernández‐Matías, Real & Pradel, 2011)—in these cases, a recruiting bird mates with the remaining territorial bird—or, more rarely, when a recruiting bird expels the former territorial bird (unpublished data). This means that Bonelli’s eagles rarely recruit into a new territory.…”