Geographical variation in song phrases differs with their function in white-crowned sparrow song

Abstract

Complex signals that convey diverse forms of information may face conflicting pressures on their structure. Certain messages, such as species identification or ‘alerting’ receivers may require a relatively invariant signal structure, while messages about dialect or individual identity and motivation require structural diversity within and among individuals of a species. A resolution to this conflict is to encode different messages in different parts of the signal. When the signal is learned, as in birdsong, parts of the signal may develop along differing developmental pathways in order to produce the necessary signal variation. I tested the hypothesis that three phrases in the song of the Puget Sound white-crowned sparrow, Zonotrichia leucophrys pugetensis, that are inferred to convey different messages will vary in their patterns of geographical variation along a 560 km long transect of the Pacific northwest coast of North America. I measured acoustic features of the songs of 267 males and tested for geographical structure using Mantel tests and Mantel correlograms. As predicted, the introductory whistle phrase, inferred to have an ‘alerting’ function, was geographically invariant. In contrast, the note complex and trill phrases, which convey information about individual identity and geographical origin, both decreased in similarity between males as distance increased. The two phrases have somewhat independent patterns of geographical variation. I suggest that differences in how these phrases develop, as measured in laboratory song-learning experiments, coupled with dispersal may contribute to the differing distributions.