A body of research by Russell Greenberg, Glenn Tattersall, and their colleagues has proposed a corollary of Allen's Rule: that in freshwater‐limited environments, bill surface area increases with temperature. Increases in both population density and sexual dimorphism, however, could also explain increases in bill surface area. After controlling for the effects of a hybrid zone, we tested whether temperature or population density in the Saltmarsh Sparrow (Ammospiza caudacuta), a sexually monomorphic estuarine specialist, explained greater variance in bill surface area. This allowed us to examine multiple potential selective mechanisms underlying the Greenberg‐Tattersall Corollary. We found that Saltmarsh Sparrows follow the general pattern of the Corollary (larger bills in warmer summer climates) but only after controlling for population density. The relationship between bill surface area and temperature varied inversely with population density. We discuss the relative abilities of sexual selection and ecological competition to explain these results.