Adaptation to Single Resources and the Evolution of Crossbill (Loxia) Diversity

Abstract

I quantitatively test the hypothesis that four taxa or "types" (species or subspecies) of Red Crossbills (Loxia curvirostra) in the Pacific Northwest have diversified morphologically in bill characters in response to alternative adaptive peaks presented by their food: seeds in conifer cones. Hypothetically, each adaptive peak corresponds to one conifer species whose seeds are (1) produced regularly from year to year, (2) held in cones through late winter when seed is most limiting, and (3) protected from depletion by potential noncrossbill competitors. Four such conifers, termed "key conifers," are present (Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Pinus contorta var. latifolia). I use data on foraging efficiency for 31 captive crossbills of four types to determine the optimal bill size and palate structure for foraging on the key conifers. As predicted, if each type is adapted for foraging on a key conifer, the observed morphology of a given type is often the predicted optimal size morphology of foraging on its respective key conifer. Two of the types have mean bill sizes (bill depth) equalling their predicted optimal size. For one of the remaining types, the observed differs from the optimum by 0.4 mm; I was unable to predict an optimal size for the remaining type. Optimal bill size varies with season. Bill sizes corresponded more closely to the optima for winter (lean period) than for summer. Observed mean width of the palate groove, in which crossbills hold conifer seeds while the seeds are being husked, was consistently close to the estimated optimal groove width. Optimal groove width was correlated (r 2 = 1.00, n = 4) with seed size (cube root of mass), suggesting optimal groove width is determined by seed size. Overall, each crossbill type has either the optimal bill size or optimal husking groove width, or both, for foraging on their key conifers. Fitness set analyses indicate that there are substantial trade—offs in foraging efficiency. The best phenotype for foraging on one conifer is often only one—half as efficient on other conifers. All four fitness sets are concave, implying selection against intermediate phenotypes. I conclude, first, that reliability of seeds on key conifers during periods of food scarcity is a critical feature in the ecology and evolution of crossbills. Second, optimization of morphological traits occurs even in populations in highly variable environments. Third, disruptive selection against intermediate phenotypes is likely. This should maintain, if not reinforce, the distinctiveness of types. Fourth, the diversity of cone structure and seed size among key conifers is ultimately responsible for the diversification of crossbills.