Female Galapagos warblers sing often - but not for the reasons anyone expected

University of Vienna

What is a female bird singing for, if not to defend her territory or intimidate rivals?

That question sits at the center of a new study published in Animal Behaviour by researchers from the University of Vienna and Anglia Ruskin University. Working on Floreana Island in the Galapagos archipelago, the team documented frequent singing by female Galapagos yellow warblers (Setophaga petechia aureola) - then systematically tested the two most commonly cited explanations for female birdsong. Neither held up.

The discovery began by accident. During a 2023 field expedition, the researchers heard an unfamiliar song that did not match anything described in previous studies or field guides for the species. The source turned out to be a female bird.

Decades of overlooking half the chorus

Birdsong research has been lopsided for most of its history. The field focused almost exclusively on male vocalizations, driven partly by assumptions that females played a passive role in sexual selection and partly by a geographic bias: most early research centered on Northern Hemisphere species, where males tend to sing far more conspicuously than females.

That picture has shifted over the past two decades. Studies across a broader range of species - particularly tropical ones - have revealed that female song occurs in more than half of songbird species. But establishing that females sing is different from understanding why. The function of female song remains one of the less resolved questions in behavioral ecology.

Two hypotheses have dominated the discussion. The first proposes that females sing for intrasexual competition - to signal aggression toward other females competing for mates or resources. The second suggests that female song functions in territorial defense, guarding resources against intruders of either sex, much as male song does.

Playback experiments on Floreana Island

To test both ideas, the team ran playback experiments. They broadcast recorded songs of males, females, and duetting pairs through speakers placed in warbler territories, simulating intrusions during both breeding and non-breeding seasons. Then they watched what the resident birds did.

The researchers recorded singing responses, measured aggressive behaviors - physical approaches, wing displays, attack flights - and tracked territories across multiple years to see whether song or aggression predicted territory retention.

The results were clear but unexpected. Females showed strong aggressive responses to simulated intruders during the non-breeding season. They approached speakers, displayed, and sometimes attacked. They also sang and participated in vocal interactions during these encounters. But - and this is the key finding - their singing had no statistical link to their aggressive behavior.

"Male song was closely linked to aggression during territorial encounters," said first author Alper Yelimlies from the Department of Behavioral and Cognitive Biology at the University of Vienna. "In females, however, singing and aggression appeared to be unrelated behaviours."

The duet pattern

Another striking observation: females rarely sang alone. The vast majority of female vocalizations occurred as duets with their paired mates, typically initiated by the male. This pattern held across both breeding and non-breeding seasons.

Female song also clustered heavily in the non-breeding season rather than during breeding, which runs counter to what you would expect if the primary function were competing with other females for mates. During the breeding season, when intrasexual competition should be most intense, female singing was less frequent.

The territorial defense hypothesis fared no better. If female song functioned as a territorial signal, you would expect singing during intrusion simulations to correlate with aggressive behavior and with territorial outcomes. It did not. Aggressive females were not necessarily singing females, and singing did not predict whether a pair retained its territory.

Communication between partners

Having ruled out the two leading explanations, the researchers propose a third possibility: female song in this species may function primarily as communication within the pair bond.

"Most female songs occurred as duets with their paired mates, suggesting that they may function in communication within the pair rather than as a territorial signal," Yelimlies said. "Studying female song is therefore essential for a complete understanding of how vocal communication evolves in birds."

Duetting in birds has been studied in other species and can serve multiple functions: coordinating pair activities, reinforcing the pair bond, or jointly signaling to neighbors that a territory is occupied by a committed pair. The Galapagos warbler data are consistent with any of these possibilities but do not yet distinguish among them.

A single species, unresolved questions

The study has clear limitations. It examines one subspecies on one island. The Galapagos yellow warbler is a sedentary tropical species, and its vocal ecology may differ substantially from migratory temperate-zone warblers. The playback experiments, while well-designed, capture behavioral responses over short time windows and may miss subtler or longer-term effects of song on social dynamics.

The sample sizes for some comparisons are modest, as is typical for field studies on island populations. And the finding that female song is not linked to aggression or territory defense does not explain what it is linked to - the pair-communication hypothesis is reasonable but remains untested through direct experimentation.

What the study does accomplish is methodical elimination. For this population, two of the most commonly invoked explanations for female birdsong do not hold. That narrows the field and redirects attention toward hypotheses - pair-bond maintenance, joint territorial signaling, or functions we have not yet considered - that may better account for why female warblers sing.

For a field that spent decades not even listening to female birds, the question itself represents progress.