Complex spatiotemporal variation in processes shaping song variation

Abstract

Abstract Understanding factors that contribute to song divergence bolsters our understanding of signal evolution and reproductive isolation. Hybrid zones often occur across environmental gradients; as such, they are excellent places to examine how signals diverge and how differentiation is maintained. We studied song variation across two hybrid zones, one old and one recent, of Tufted Titmouse (Baeolophus bicolor) and Black-crested Titmouse (B. atricristatus), across an environmental gradient where the two titmouse populations meet. In the recent zone, noise and vegetation structure were correlated with several song characteristics, but in the older zone, these features did not correlate despite similar gradients in song features. Our data, combined with previous studies, suggest that despite overall similarities in characteristics, songs in the older zone may be more shaped by sexual selection, whereas songs in the young zone are shaped by environment. Thus, even within the same species, processes shaping signal structure can vary spatially and temporally.