Light-level geolocators confirm resident status of a Southern European Common Crossbill population

Abstract

Common Crossbill (Loxia curvirostra) populations are dependent on specific key conifer species for their primary food supply, but many of these conifers standardly show irregular cone crop production. Consequently, the species has evolved a nomadic behavior, breeding in areas with a high crop production in given years. Northern European crossbills mostly forage on Norway Spruce (Picea abies) and migrate along a northeast–southwest gradient across Europe. In contrast, crossbills from Spain were considered to be resident until recent studies on crossbill populations in the Pyrenees showed that a fraction of the population may also migrate. The type of movements of these Spanish birds, however, remains virtually unknown. The aim of our study was to resolve the question of whether these birds displace on a small local scale or whether they actually move to a distinct non-breeding area for part of the annual cycle. We tracked 14 male Common Crossbills from the Pyrenees with light-level geolocators for an entire year. Our results suggest that these birds were resident; only one bird may have shown a west–east movement of up to 200 km along the Pyrenean axis. We conclude that movement of the Common Crossbill from the Pyrenees may predominantly consist of relocation to nearby mountain ranges in Iberia or vertically in altitude. We expect that such small-scale movements have implications for population dynamics, and we recommend further research with telemetry to resolve the details of this small-scale movement.