Restringido

Abstract The impact of nomadism on animal populationdynamics (e.g. survival) is poorly understood. This factor isnot inconsequential because our current view on animalsurvival rates could suffer from biases if it is ignored. Thedegree of nomadism in Common Crossbills (Loxia curvi-rostra) is reported to differ population-specifically, andthus they constitute a good model for the study of survivalin nomadic species. Southern Europe hosts resident birds,probably also local vagrant birds and non-local, vagrant,northern European Crossbills. The impact of vagrants onthe estimation of apparent survival rates has never beendetermined in areas where Crossbills have been reported asresident. We hypothesise that transients (birds for whichsurvival from year tto year t?1 is zero) will be detectableif a majority of vagrants remain at a particular site for just afew months. Alternatively, if vagrants remain for longer,transients will be absent by definition, so apparent survivalestimation will be lower than would be if transients did notexist. As wing length in vagrants is commonly longer thanin residents, we can expect a negative effect of wing lengthon survival, as long-winged (vagrant) Crossbills emigratefrom the area after a few years, thus demonstrating lowerapparent survival rates than resident, local birds on a localscale. Alternatively, if vagrants have a negligible impact onthe estimation of local apparent survival rates (i.e. if resi-dents clearly outnumber vagrants), we should not detecttransients nor find any effect of wing length on survival. Totest this hypothesis, we used data collected over a period of16 years at a site located in a Scots pine (Pinus sylvestris)forest in northern Iberia. Capture–recapture data wereanalysed with Cormack–Jolly–Seber (CJS) models.Although transients were not detected, the CJS modelsshowed that survival was negatively affected by winglength. Our results support the hypothesis that vagrantsoccurred in the area for more than 1 year before subse-quently disappearing, and that their presence has a strongimpact on local survival estimation. Accordingly, if thepresence of vagrants is not considered, this can lead to theunderestimation of local survival rates of resident Crossbillpopulations.

Alonso, D., Arizaga, J. 2012. The impact of vagrants on apparent survival estimation in a population of Common Crossbills (Loxia curvirostra). J.Ornithol. 154(1): 209-217.