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.
Once its first mate lays eggs, a male pied flycatcher will often set up another nest several hundred meters away and court passing females. The stakes are high: a male splitting its time between two nests provides less help raising chicks. Earlier research showed that most females prefer to settle with an unmated male.
The team recorded 17 wild males twice — once while a male was a bachelor, and again after it had mated and moved to a second territory. Using bioacoustics software, they found 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 a "territorial 'Keep Out' signal" aimed at rival males, says Stephen Nowicki of Duke University, who was not involved in the work. Shorter songs, on the other hand, may be a tactic to elude the first mate: previous research has shown the first female can recognize her 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."
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 still put in a word for the males, though: calling them "cheaters" risks making them sound like tiny Machiavellians when they are not actually setting out to deceive. "It's just selection acting on them to increase their reproductive success."
