Disentangling the origin of crossbills using morphology and isotopic (δ2H) characters. Are southern European crossbills restricted to population-specific key resources?

Abstract

Specialist foragers depend on specific prey commonly associated with limited habitat, which is often patchily distributed. Understanding how specialists use habitat patches is important to their conservation. Crossbills (Loxia spp.) are one of the best examples of a foraging specialist, because they exploit population-specific conifer species. In this work, we used morphology and stable isotopic analyses (δ2H) to test the use of two different key conifer species (Pinus spp.) by crossbills. This study was conducted in Spain to test whether a small patch of Aleppo pine (P. halepensis) from the Ebro Valley hosted native, resident common crossbill (L. curvirostra) populations or, by contrast, if it was used just as a “stopover” or a passage site between the two main mountain ranges situated to the north and south of this valley, which is mostly occupied by Scots pine (P. sylvestris). Crossbills caught at the Ebro Valley used this zone only temporarily, thus supporting the lack of a stable, strictly resident population. Morphological and isotopic analyses revealed that these birds were likely to belong to an Aleppo pine-associated population, and likely not to Scots pine crossbills moving between the Pyrenees and the Iberian System. Therefore, we observed evidence supporting high foraging specialization and population-specific use of key conifer resources in Spain. This work highlights the usefulness of combining morphological and stable isotopic analysis to infer the origin and possible movement patterns of crossbill populations.