A critical season approach to Allen's rule: bill size declines with winter temperature in a cold temperate environment

Aim: Bill size is positively correlated with average air temperature in several avian taxa. The conventional explanation for this pattern is that the poorly insulated surface area of the bill functions to dissipate excess heat in hot climates or retain heat in cold climates (Allen's rule). We hypothesize that the applicability of Allen's rule depends upon the season of critical thermal stress, which may vary by location and species. We previously found that bill size increases with high summer temperature in song sparrows (Melospiza melodia) of California, supporting the hypothesis that larger bills are an adaptation to release dry heat and minimize evaporative water loss in a hot, dry environment. Song sparrows in eastern North America face a different climatic regime, in which summers are warm but moist, and winters are cold. We predicted that this climate region would select for smaller bills to minimize heat loss. Location: From coastal Delaware and Maryland to the Allegheny Plateau of the eastern USA. Methods: We measured body size and bill morphology of 274 song sparrow individuals across a climatic gradient and geographical gradient. We used linear regression models to test for effects of geographical location and 30-year means of minimum winter and maximum summer temperatures and annual rainfall on bill size. Results: Bill size was best predicted by, and positively correlated with, cold winter temperatures and distance from the Atlantic coast. Body size did not change substantially with distance from the coast, indicating that the pattern of bill size was not a result of allometry. Coastal birds had bills larger than predicted by a linear effect of minimum temperature. Main conclusions: Collectively, these results support the hypothesis that variation in bill size reflects selection for winter heat retention rather than for summer heat dissipation. Bill size may be shaped by physiological responses to regional climates, and the season of critical thermal stress may vary geographically, even on relatively small spatial scales.