The impact of vagrants on apparent survival estimation in a population of Common Crossbills (Loxia curvirostra)

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

“…Long‐winged crossbills did not show more negative isotopic values suggesting a northern origin. This result is intriguing because we found earlier for the same sampling area and a longer study period that long‐winged crossbills appeared more likely to be northern crossbills, as they tended to have lower apparent local survival rates and a color pattern more typical for northern European crossbills (Alonso & Arizaga, ). Perhaps, the numbers of these northern birds within our population are higher during other irruption years not included in this study.…”

“…Given the presumed overall residency of southern crossbills, it was hypothesized that these more temporary members of the population are northern crossbills. Indeed, for our population, we documented evidence of long‐term (>1 year) presence of presumably northern crossbills (Alonso & Arizaga, ), although genetic studies revealed that these northern birds differ genetically from those found in Spain, suggesting that most of them must stay for shorter periods and do not reproduce (Björklund, Alonso & Edelaar, ). However, because morphology alone is insufficient to identify birds as being northern or southern crossbills, it remains unclear whether these local population fluctuations are entirely because of the irruptions of northern crossbills or whether southern birds from elsewhere could also enter and leave the population.…”

“…Therefore, year‐associated variations in the number of captures would show that local crossbill densities differ among years. A 6.6% of all captured crossbills are recaptured in subsequent crossbill years, suggesting that a substantial part of the population is resident (Alonso & Arizaga, ). Therefore, we infer that years with a higher number of captures per day represent years in which influxes of crossbills from elsewhere have occurred.…”

“…P1 within our region is replaced mostly before June (Alonso & Arizaga, ), so northern crossbills reaching the Pyrenees during the second half of the year (Newton, ) should have replaced P1 in areas further north. However, if some of these irruptive birds stay >1 year in the area (Alonso & Arizaga, ), these will moult the P1 in the Pyrenees and, therefore, show local isotopic signatures. However, these long stays seem to be rare within the region, as shown by genetic studies (Edelaar et al ., ; Björklund et al ., ), and hence the effect of this bias should be minimal.…”

“…Thus, the presence of northern crossbills in Iberia is well documented (Cramp & Perrins, ; Borras & Senar, ; Newton, ). A recent analysis of apparent survival in crossbills in the Pyrenees has shown that many birds are resident (Alonso & Arizaga, ), indicating that a resident population occurs, presumably adapted to utilize Scots pine Pinus sylvestris (Alonso et al ., ; Edelaar et al ., ). Nevertheless, the population in this area fluctuates a great deal, suggesting that birds from elsewhere also enter the local population every now and then.…”