Why some birds sing elaborate songs in the winter

Male great reed warblers may be practicing how to woo

Marjorie Sorensen with birds in Zambia

WHY SING IN WINTER  Marjorie Sorensen, tracking bird movements in a seasonally flooded valley in Zambia (left), is studying why great reed warblers (one shown, right) overwintering there sing so much.

L-R: Jason Boyce, M. Sorensen

Male birds’ puzzling off-season singing in winter could be practice for flirting in spring.

Europe’s great reed warblers (Acrocephalus arundinaceus) and some other male long-distance migrants sing extensively when overwintering in sub-Saharan Africa, says Marjorie Sorensen, now at Goethe University in Frankfurt. “Why are they doing this when they’re thousands of kilometers from any possible breeding opportunity?”

Singing seems costly. Reed warbler songs — “very harsh-sounding and creaking,” Sorensen says — are changeable compositions made up from a male’s repertoire of some 40 to 60 sounds. Tests find that singing demands about 50 percent more energy than reed warblers spend resting. Plus, singing cuts into foraging time and risks catching predator attention.  

Biologists largely expected the wintertime musical extravagance to be territory defense. But the birds creak variations of courtship serenades instead of shorter territorial anthems, Sorensen and her colleagues report in the March American Naturalist. And instead of squabbling over their patches, the warblers freely crisscross the landscape. Nor did testosterone elevation left over from the previous breeding season play a role — levels had dropped to winter lows.

As an alternative explanation, Sorensen and colleagues suggest that wintertime song is practice. Female reed warblers prefer males that deliver more complex songs. And among bird species overwintering in Africa, those that sang the most were also likely to show signs of strong evolutionary pressure on musical courtship. Their songs were especially complex, but their plumage didn’t show much difference between the sexes. When music matters so much, practice could, too.

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