Aims: Several studies have shown that different crossbill (Loxia curvirostra) populations inhabiting habitats dominated by different conifer species differ in bill morphology. The aim of this paper is to compare the bill morphology of crossbills inhabiting mountain (Pinus uncinata) and black pine (P.nigra) forests within the same geographical area in the Mediterranean where there is a high diversity of native pine species. We predicted that the large spines and thick apophyses of the mountain pine should favour crossbills with larger and more massive beaks.
Location: Mountain pine crossbill populations were sampled at 6 localities within the Port del Comte mountain (SolsonÁ¨s, Lleida) and Lles areas (Cerdanya, Lleida). Black pine crossbill populations were trapped at 8 localities located between the Oliana and the Busa mountain ranges (SolsonÁ¨s) and Castelltallat mountains (Bages, Barcelona).
Methods: Measurements of head length, bill length, depth and width, and the distance between the tips of the lower and upper mandibles, from a sample of 1,152 birds captured at mountain pine populations and 132 at black pine populations.
Results: Crossbills inhabiting mountain pine areas showed more massive bills and greater bill depths than those inhabiting black pine areas. The difference found in the bill depths approached 0.2 mm, which is similar to the differences between reproductively isolated call types of crossbills in other areas in Central Europe and North America. An index of bill depth / bill length also provided very significant differences between birds of different habitats.
Conclusions: Results support the hypothesis that crossbills have adapted locally to the cones of these two pine species or, alternatively, that such differences have arisen from differential dispersal whereby individuals sort between habitats based on their bill sizes.
