‘Cheating’ birds change their tune

To avoid ending up with a less committed mate, some female birds listen for red flags in their suitor’s song.

Male pied flycatchers change their song after mating, and females use these song features to tell if a male is already paired, researchers report July 1 in Ethology. The results are consistent with earlier research showing that most female pied flycatchers prefer to settle with an unmated male.

“The female doesn’t really care about the song,” says behavioral ecologist Stephen Nowicki of Duke University, who was not involved in the work. “She cares because the song is a reliable indicator of some quality of the male,” including whether it already has a mate.

Once its first mate lays eggs, a male pied flycatcher (Ficedula hypoleuca) will often set up another nest several hundred meters away and court passing females. The stakes are high for the females, as a male splitting its time between two nests provides less help raising chicks.

Ethologist Helene Lampe and her colleagues recorded 17 wild males twice — once while a male was a bachelor, and again after it had mated and moved to a second territory to woo another female. Using bioacoustics software, the team found that two things changed. The songs got shorter — from about two seconds to about 1.7 — and the male repeated itself less.

“Usually, a male will cycle through his song syllables in an orderly manner — one or two syllables are usually repeated from one song to the next,” says Lampe, of the University of Oslo. “But when they become polyterritorial, they do it much less often.”

The decreased syllable repetition may be geared toward rival males — a kind of “territorial ‘Keep Out’ signal,” Nowicki says. This fits the results of an earlier study where males tended to repeat syllables less near a rival male, but not near a female.

Shorter songs, on the other hand, may be a tactic the male uses to elude its first mate. Previous research has shown that the first female can recognize its partner’s song and will fly over to chase off other females, so shorter songs may lower the male’s chances of being caught “cheating,” the researchers speculate.

To test whether females can hear the difference, the team played recorded songs from both single and mated males in large outdoor aviaries. Most females (21 out of 29) made more visits to the nest box beside the speaker playing a bachelor’s song. But that preference disappeared when researchers doctored the bachelors’ songs to resemble that of a mated male.

Nowicki would like to put in a word for the males, though. Calling them “cheaters” risks making them sound like tiny Machiavellians, he says, when they’re not actually setting out to deceive the females. “It’s just selection acting on them to increase their reproductive success.”