Female fairy-wrens sing for other females
IN AUSTRALIA, FEMALE fairy-wrens sing to communicate with other females, rather than to attract a mate, a new study suggests.
Birdsong plays an important role in the study of bird communication, mating behaviour and sexual competition. However, until recently, it was believed that male birds did most of the singing, and mainly for the purpose of attracting females.
Dr Kristal Cain, a researcher at the Australian National University in Canberra and lead author of the study, says that when scientists observed female birds singing, it was previously dismissed as a freak occurrence. “It’s a great example of how assumptions can colour your observations,” she says.
For this study, published in Animal Behaviour in September, Kristal and her team observed 75 male and 75 female superb fairy-wrens (Malurus cyaneus), and compared male and female spontaneous birdsong, as well as how each sex reacted to each other’s singing.
They found that, when confronted with unfamiliar female songs, females sang in reply much more frequently than males. This suggests that the most important role of female singing is female-to-female competition over things like breeding territory, rather than to attract mates.
Singing birds aren’t always male
“These kinds of data are crucial for understanding why song evolved,” says Kristal. “In the past, birdsong has been studied almost exclusively in males, which gives us a very biased view of the function and evolution of song.”
This bias was a fair assumption in more temperate climates, where females of many species have lost the ability to sing. However, birds in the tropics and the southern hemisphere are a different story, and Australian studies including this one have revealed female birds vocalise just as much as males, if not more.
Dr Eugene S. Morton, an evolutionary biologist from York University in Toronto, Canada, agrees that the study of temperate birds has produced a ‘terrible bias’ towards assuming birdsong has everything to do with mating.
“This is why reports such as this one are so valuable – they reset our thinking and correct the bias,” Eugene says. “We realise that birdsong in temperate birds, usually restricted to males and breeding, is the odd man out, not the norm, so we are finally able to grasp the evolutionary significance of it.”
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Why do birds sing?
The finding raises an important question: if female birdsong provides an advantage in female-female communication, why have female birds in temperate climates lost the ability to sing?
Kristal says that a key to this mystery might lie in the process of seasonal migration, commonly seen among bird species in Eurasia and the Americas. As males typically arrive at breeding sites first, female-female competition of breeding sites may have become redundant.
“Interestingly,” Kristal notes, “females also appear to lose bright colours when they start migrating, suggesting colour is also an important signal used in female-female competition.”
Kristal says this research is just the tip of the iceberg, as more study on Australian songbirds is needed to fully understand why song evolved.
“Now we know that songbirds probably evolved in Australasia, and that the ancestral female songbirds also sang,” says Kristal. “That simple fact changes our perspective a lot.”
