Ecology shapes birdsong evolution: variation in morphology and habitat explains variation in white-crowned sparrow song.

Ecological variation appears to underlie the evolution of mating signals in many taxa, yet understanding of how this process occurs over time is limited. Here, I investigate whether changes over time in a well-studied mating signal-birdsong-are attributable to ecological factors that affect signal production and transmission. Variation in the acoustic properties of songs is thought to be affected by the mechanics of sound production as well as by features of the habitat that affect sound transmission. To determine whether these mechanisms contribute to song variation, I compare patterns of morphological and habitat variation with variation in song structure among populations of white-crowned sparrows (Zonotrichia leucophrys) at two time points separated by 35 years. Among contemporary (2005) populations, vegetation density and bill size explain significant variation in song structure. The direction of change in song structure between 1970 and 2005 is also consistent with the direction of change in vegetation density. These findings suggest that variation in factors that affect signal production and transmission explains significant variation in white-crowned sparrow song.