For some female birds, the best mate is one with a big vocabulary. At first blush this may seem like an odd way to pick a partner. After all, a male with more territory will be better able to provision a nest, and a male who can sing longer is probably healthier. So why should a song's complexity matter? In this month's Animal Behaviour, a team of biologists propose an answer: Birds with a greater repertoire of song "syllables" are less likely to carry parasites.
Evolutionary biologist Kate Buchanan of the University of Stirling in Scotland observed the improbable connection in the sedge warbler, a bird that winters in Africa then flies north to England in April to breed. The males hit British soil first to stake out their territories. When the females show up, the males attract them by "song flighting," an elaborate display in which they burst up 4 meters into the air and parachute down, singing as they go. Each song typically consists of seven or eight repeated syllables, but a male's vocabulary may run to as many as 70.
Buchanan and three colleagues drew blood samples from 73 warblers to test for parasites and recorded and analyzed songs. They expected parasites to diminish the amount of time spent song flighting, says Buchanan, because a parasitized bird would have less energy. However, she says, "we were surprised that there was a much stronger connection with the complexity of the song." Birds that were parasite-free had a greater variety of syllables (65, on average) in their vocabularies than birds with parasites (55). Because a sedge warbler learns most of its song repertoire early in life, the findings suggest that female warblers use song complexity to gauge not the male's current health, but his health and parasite resistance since childhood, Buchanan says.
"We've known that females prefer males with a complex song repertoire," says William Searcy, a behavioral ecologist at the University of Miami in Florida, "but the reason for that preference has been particularly puzzling." The sedge warbler joins a growing list of species, including barn swallows and rock doves, of which females are known to find parasite-free males sexier. However, in most of those birds the plumage advertises health; the warbler is the first example of a bird that proclaims its health through song.