Stable isotopes in a southern E uropean crossbill population indicate restricted movement between regions with different pine species

Abstract

The adaptation of irruptive birds to breed where they find abundant food allows them to be highly mobile and ubiquitous and opens a debate on the ecological and evolutionary impact of this behavior. Using δ2H analyses, our aim here was to test whether the density fluctuation in a southern European crossbill population is caused by movements of irruptive northern European crossbills or of crossbills from nearby. Sampling was carried out in the western Pyrenees during four consecutive breeding seasons (2009–2012). During these four years, 2009 was a year with a much higher number of captures per day and this coincided with an invasion of crossbills in northern Europe. Only two out of 160 crossbills showed clear isotopic signatures of a northern European origin (and both were caught in 2009). Moreover, in 2009, we recaptured a bird ringed in The Netherlands. Otherwise, we found no differences among years in δ2H and no significant effect of wing length (indicative of a northern origin) or residency status (long-term recaptures vs. non-recaptures) on δ2H. Wing length also did not vary among these four years. Therefore, our results suggest that annual variation in local crossbill abundances is not only due to the movements of northern irruptive birds, but also to a large part due to the influx and local reproduction of individuals of Iberian origin. The lack of annual variation in isotopic signatures and similarity with values from a nearby mountain area with the same food resource (the pine Pinus sylvestris) suggests that movements are primarily between areas with the same type of resource. Thus, our results support recent morphological and genetic studies that indicate that crossbill populations within Iberia are not structured geographically but by resource use.