Black pine cone crop size, cone structure and pollination failure affect the amount of seed predation by crossbills in the Iberian Peninsula

Understanding the causes of geographic variation in traits is the focus of much research. Prior studies on multiple species of cone-bearing conifers have discovered that geographic variation in cone structure is related to variation in the intensity and form of natural selection exerted by seed predators. Here, we used data on the intensity of pre-dispersal seed predation by common crossbills (Loxia curvirostra) in relation to black pine (Pinus nigra) cone traits, along with seed predation and cone trait data for nine sites in the Iberian Peninsula to determine whether patterns of differential seed predation could account for among-site variation in cone traits. The tree and cone preferences of crossbills within a site were similar to those found in other studies of crossbills and conifers: crossbills preferentially foraged on trees producing many small cones with small scales. In contrast, the variation in the intensity of seed predation among the nine sites appeared to be negatively influenced by the proportion of empty seeds. The preferences for small, thin-scaled cones within a site have the potential to cause the evolution of larger and thicker scaled cones, because such cone traits are often heritable. However, the different intensities of seed predation among sites that we document are unlikely to lead to concordant trait differences among sites because the proportion of empty seeds appears related to environmental factors (e.g., precipitation) affecting pollination success that vary from year to year. Controlling such among-site variation is key for inferring whether selection can account for geographic variation.