A study in this week’s Proceedings of the Royal Society B suggests that the sexually dimorphic pattern of birdsong we’re used to in temperate latitudes — with males singing elaborately and females usually not — evolves because female birds stop singing when their species move to more northerly latitudes [$-a]. Why this is, however, remains an open question.
Photo by Vicki & Chuck Rogers.
The study’s authors reconstruct the evolution of home range (temperate versus tropical) and sexual song dimorphism (both sexes singing versus only males singing) in the New World blackbirds, the family that includes orioles, cowbirds, and red-winged blackbirds (pictured). The reconstruction reveals a strongly significant association between the evolution of male-only singing and transitions from tropical to temperate breeding ranges. The authors discuss this transition in a few key groups, including North American red-winged blackbirds (Agelaius phoeniceus) and their sister species, the Cuban red-shouldered blackbird (A. assimilis):
Females of A. assimilis are nearly indistinguishable from conspecific males in song structure and song rate and are also similar in plumage and body size … whereas females of A. phoeniceus differ considerably from conspeicific males in these traits …. it is clear that the changes in female song and plumage must have occurred quite rapidly. [In-text citations omitted.]
As clear as the observed pattern is, however, there doesn’t seem to be a good general explanation for it. The authors point to cases where female singing is lost within tropical-breeding lineages, which might help disentangle the effects of latitude and other evolutionary forces generating the observed pattern. In these cases, loss of female song is associated with colonial nesting and polygynous breeding, whereas singing by both sexes is associated with year-round pairing.
The temperate-breeding blackbirds tend to be migratory, with males often arriving at the breeding range ahead of females to establish nest sites and territories. In these cases singing by the males serves to attract females and to announce ownership of territory. Could that migration-induced division of labor lead females to give up singing? I’m just an amateur birder, but it sounds plausible to me.
Price, J., Lanyon, S., & Omland, K. (2009). Losses of female song with changes from tropical to temperate breeding in the New World blackbirds Proceedings of the Royal Society B: Biological Sciences, 276 (1664), 1971-80 DOI: 10.1098/rspb.2008.1626