Male pied flycatchers subtly alter their songs after mating, and female birds appear capable of detecting those changes to avoid pairing with a male that is already committed elsewhere, according to a new study published in Ethology.

The research, led by scientists at the University of Oslo, provides fresh insight into how birds use vocal signals to navigate the complex reproductive choices they face. Rather than suggesting that the birds deliberately deceive one another, researchers say the changes in song are more likely the result of natural selection acting on males as they attempt to maximize their reproductive success.

The study, titled “Female Pied Flycatchers Ficedula hypoleuca Can Discriminate Between Songs of Unmated and Mated Males and Avoid Being Deceived Into Polygyny,” examined how female pied flycatchers respond to subtle differences in the songs of unmated and mated males.

Pied flycatchers are small migratory songbirds found across much of Europe and parts of western Asia. During the breeding season, males establish territories and use song to attract females and defend nesting areas. While many males pair with a single female, some become polyterritorial, establishing a second territory several hundred metres from their original nesting site and attempting to attract another female.

Such behavior can create a conflict between male and female reproductive interests. A male with two territories may increase his opportunities to reproduce, while the females involved may receive less parental assistance because the male has to divide his time between nests.

“A classic conflict of interest between the sexes occurs when males benefit from polygynous mating whereas females benefit from monopolising male parental care,” the researchers noted in the study.

To investigate whether females could identify whether a male was already paired, Helene Lampe, an ethologist at the University of Oslo, and her colleagues recorded the songs of 17 wild male pied flycatchers at two stages of the breeding season.

The researchers first recorded the males while they were unmated. They then recorded the same males after they had found a mate and moved to a second territory, where they could potentially attract another female.

Analysis using bioacoustic software revealed subtle but consistent changes in the males’ songs. The songs became shorter, decreasing from approximately two seconds when the males were bachelors to around 1.7 seconds after mating.

The researchers also observed changes in how the birds repeated song syllables. Unmated males generally repeated one or two syllables from one song to the next in a relatively orderly pattern. After mating and establishing another territory, however, the males showed considerably less repetition.

The researchers suggest that the reduced syllable repetition could be connected to the demands of defending multiple territories. A male occupying two territories has to deal with rival males, protect nesting sites and return regularly to his original territory.

Behavioral ecologist Stephen Nowicki of Duke University, who was not involved in the research, suggested that the altered repetition pattern could function partly as a territorial signal directed at other males.

Earlier research has found that male pied flycatchers tend to reduce syllable repetition when a rival male is nearby, while the same change is not observed in the presence of a female. This supports the possibility that some of the song changes are linked to territorial competition rather than an attempt to mislead potential mates.

The shorter songs could have another advantage. Previous research has shown that female pied flycatchers can recognize their partner’s song and may fly to a second territory to drive away another female. The researchers therefore speculate that shorter songs could reduce the likelihood that a male will attract attention from his first mate while he is attempting to court elsewhere.

The team then tested whether females could distinguish between the two types of songs. In large outdoor aviaries, researchers played recordings of songs made by both unmated and mated males near nest boxes.

The response was notable. Of the 29 female pied flycatchers tested, 21 made more visits to the nest box associated with the song of an unmated male. However, when researchers modified the bachelor songs to make them resemble the songs of mated males, the females’ preference disappeared.

The findings indicate that females can use relatively small changes in male vocal behavior as information when choosing a potential partner.

The researchers, however, caution against interpreting the behavior through a human moral lens. Describing the males as “cheaters” or “Machiavellian” can imply deliberate deception and calculated intent that may not accurately reflect what is happening biologically.

Nowicki emphasized that the males are not necessarily consciously attempting to deceive females. Instead, their behavior can be understood as the product of natural selection, with individuals developing traits and behaviors that increase their reproductive success over generations.

The study therefore highlights a broader principle in animal behavior – communication signals can evolve in response to competing reproductive interests, while receivers evolve ways to interpret those signals.

For female pied flycatchers, a small change in the rhythm, length and repetition of a male’s song may provide valuable information about his current social and reproductive status. What sounds like a minor variation to human ears could therefore influence one of the most important decisions in a bird’s breeding season — choosing a mate.