Mechanisms of song divergence between swamp sparrow subspecies

Abstract

Speciation is promoted when traits under divergent selection also contribute to non-random mating. For example, divergent selection on bill morphology in birds can influence song because of the role of bills in song production (ecomorphological constraints). Since song is used by females to find appropriate mates, divergent selection on bills through its impact on song can promote non-random mating. However, because song in songbirds is learned, divergence in song between populations could be the result of cultural influences (cultural evolution). In this study, we use an experimental approach to address the relative influence of ecomorphological constraints and culture on song divergence in swamp sparrow subspecies (Melospiza georgiana). As with other songbirds, swamp sparrows are subject to cultural influences that may explain differences in song between subspecies (cultural evolution). However, ecological conditions promote larger bill size in the coastal subspecies of swamp sparrows rendering males unable to perform the broad-band, rapid songs that inland females prefer potentially contributing to non-random mating between subspecies. To determine the impact of ecomorphological constraints on song learning, we trained hand-raised swamp sparrows from divergent populations with an identical set of tutor songs from a foreign inland population. Males from both populations learned species typical songs. We found that inland males produced learned songs with better vocal performance than coastal males suggesting that observed differences in specific features of song are influenced more by morphological constraints than cultural drift. Further, we found that inland males reproduced tutor songs more accurately than coastal males suggesting that coastal males tried but were less able to accurately reproduce physically challenging songs. These results support the current model for how motor constraints influence song production and may demonstrate a mechanism by which natural selection can influence signal evolution and population divergence through divergent selection on bills.