Environmental correlates and functional consequences of bill divergence in island song sparrows

Inferring the environmental selection pressures responsible for phenotypic variation is a challenge in adaptation studies as traits often have multiple functions and are shaped by complex selection regimes. We provide experimental evidence that morphology of the multifunctional avian bill is related to climate, not foraging efficiency, in song sparrows ( Melospiza melodia ) on the California Channel Islands. Our research builds on a study in song sparrow museum specimens that demonstrated a positive correlation between bill surface area and maximum temperature, suggesting a greater demand for dry heat dissipation in hotter, xeric environments. We sampled contemporary sparrow populations across three climatically distinct islands to test the alternate hypotheses that song sparrow bill morphology is either a product of vegetative differences with functional consequences for foraging efficiency or related to maximum temperature and, consequently, important for thermoregulation. Measurements of >500 live individuals indicated a significant, positive relationship between maximum temperature and bill surface area when correcting for body size. In contrast, maximum bite force, seed extraction time, and vegetation on breeding territories (a proxy for food resources) were not significantly associated with bill dimensions. While we cannot exclude the influence of foraging ability and diet on bill morphology, our results are consistent with the hypothesis that variation in song sparrows’ need for thermoregulatory capacity across the northern Channel Islands selects for divergence in bill surface area. SUMMARY STATEMENT Island song sparrow bill differences are correlated with climate, not vegetation, and experimental evidence finds no functional effect on foraging efficiency. This suggests many factors shape this multifunctional trait. ### Competing Interest Statement The authors have declared no competing interest. * PC1bod : body size; composite score of body size based on PCA of tarsometatarsus and wing lengths used to estimate residual bill surface area PC1veg : primary axis of variation in vegetation; composite score of vegetation based on NLPCA of common vegetation types in sampling locations PC2veg : secondary axis of variation in vegetation; composite score of vegetation based on NLPCA of common vegetation types in sampling locations PC1bill : primary axis of variation in bill dimensions; composite score of bill depth, width, and length used in analysis of foraging efficiency T : maximum environmental temperature (in July) of sampling location