Selection of Montain Pine Pinus uncinoto cones by Common Crossbill Loxia curvirostra in the Pyrenees

摘要

At the subalpine level in the Pyrenees, Common Crossbills Loxia curvirostra feed exclusively on seeds of Mountain Pines Pinus uncinata. Cones of this exclusive subalpine tree species present a characteristic thick distal end of their scales (hooks). When cones are ripening from August to December when they open, crossbills forage only on a limited number of trees. They pick up and remove the closed cones from the branches and extract seeds by prying apart scales. Morphological traits and seed number of cones selected by crossbills were compared to those of cones from surrounding unpredated trees. Crossbills preferred smaller cones with thinner hooks and more seeds, presumably to limit energy expenses and to increase feeding performance. Comparatively with Northern crossbills species, Parrot and Scottish Crossbills, feeding on smaller cones of Scot Pines, Common Crossbills in the Pyrenees feed on bigger cones with higher distal scale thickness, but have a smaller bill. Earlier ripening and opening of the cones of Mountain Pines (in late November, making avaible high energetic seeds early in winter) regarding to the cones of Scot Pines which shed their seeds in May in Northern Europe may explain that a stronger bill would not be advantageous in the Pyrenees. Phenology of cone ripening seems to be the main selective agent independent of the cone structure acting against a stronger bill. Nevertheless as cone traits are known to evolve in response to crossbill predation, selection by crossbills may favour Mountain Pines that produce larger cones with thicker hooks, and reciprocal adaptation may occur, cone structure driving selection on bill traits of Pyrenean crossbills (in comparison with Alpine crossbills feeding on other conifer species) in a coevolution process.