While singing attracts mates in Europe, it makes much less sense over the winter in Africa. For one, it burns calories. Second, it tempts predators. Those costs might be worth it if it meant attracting female attention, but that’s not happening in winter.
In the past, ornithologists have explained away winter singing in many different species as territorial posturing. Or as an accidental byproduct of the birds building up testosterone to prepare for the breeding season to come. Now, a new study led by Spottiswoode’s former grad student Marjorie Sorensen argues for a much cuter explanation: The birds are rehearsing for next summer.
Sloshing through tall grasses in Zambia’s wetlands, Sorensen’s team tested the three competing theories. If the birds were singing to protect their territories, playing back a pre-recorded song would provoke aggression, singers wouldn’t want to share the same space, and the songs would be short missives like the ones warblers use to warn away invaders in Europe. None of these predictions came true.
Rejecting the hormone theory was even simpler. If the birds were singing because they were jacked up on testosterone, singers caught in a mist net would show higher testosterone levels than the roughly half of males who didn’t sing—but once again, they didn’t.
Next, they tested the “rehearsal” hypothesis. By pointing a long-distance microphone at singing birds, Sorensen recorded their calls, graphed them, and found that they looked more like summer breeding songs than short territorial warnings. But with one crucial difference.
Though the African songs are just as complex, they follow a slower tempo, and they stretch on and on. “They don’t really have breaks between their songs,” Sorensen says. “They’re just singing, singing, singing.” The songs also seem to meander mid-verse, with the birds switching more rapidly from sound to sound.
Sorensen thinks that might be because they are working through material that isn’t yet crowd-ready, instead of trotting out old standards. In the midst of cutthroat competition on the breeding grounds, birds tend to repeat complex phrases insistently, as many as four of five times. That doesn’t happen in Africa. “If a female is there, and you know you have a very complex syllable, you want to make sure that she hears that syllable,” Sorensen says. But when no one’s listening, flipping through different sounds might be a better way to learn.
Reckoning that African singing might be more important to birds that value song when picking mates, Sorensen’s team also combed through the literature for mentions of winter singing in 57 other migratory birds. They found that two types of birds were the most enthusiastic winter vocalists: those in which males sing more complex songs, and those in which males are drab, and need to use songs instead of colorful plumage to lure females. In both cases, sex comes from songs, and better songs, Sorenson and Spottiswoode argue, come from practice.