Many Female Songbirds Have Shorter Lifespans Than Males, Scientists Find

Female birds have long flown under the radar. In birding, a focus on often flashier males has often skewed our perceptions of the avian world, and in science, little was known about the females until relatively recently. For example, ornithologists once believed that female birds rarely if ever sang, but research in the past decade has revealed that female birdsong is more diverse and complex than previously thought. While these biases aren’t always intentional, they influence conservation and scientific research nonetheless. 

Now, a new study showcases yet another reason why we should pay greater attention to female birds. In a paper published in July in the journal Proceedings of the Royal Society B, scientists found that the lives of female perching birds—the biodiverse taxonomic group that contains all songbirds and flycatchers—are, on average, shorter than those of males. 

To arrive at their discovery, the team analyzed more than 560,000 bird-banding records collected in the United States and Canada over nearly three decades, making it the largest study of its kind. Among the perching bird species included, males showed a 12.9 percent higher average survival rate than females.  If they made it to adulthood, males lived about 1.66 years—that’s 19.3 percent longer than the 1.39-year average lifespan of adult females.  

“I was just really surprised that this pattern was so consistent for so many species over almost a 30-year period,” says lead author Joanna Wu, an ecologist at the University of California, Los Angeles who studies female bird biology. 

Wu and her co-authors relied on records from 1,229 banding stations operated by the Monitoring Avian Productivity and Survivorship (MAPS) program, an initiative of the Institute for Bird Populations. At these stations, individual birds are safely captured in mesh mist nets, marked with bands around their legs, and (hopefully) recaptured at a later time for follow-up study. The resulting data revealed lower female survival for  82 out of 92 focus species, but in particular, the pattern became significantly clear for 35 species across several passerine bird families—from titmice and chickadees to thrushes and cardinals.

According to Steven Austad, a longevity researcher who was not involved with the study, the finding is a major one. “There have been previous studies that have reported that female birds live shorter than males in some species, but this is by far the biggest study with the most birds and species involved,” says Austad, the scientific director of the American Federation for Aging Research. “It will be quoted a lot in the future literature.”

The authors of the paper also hypothesized why those sex differences exist. One possible cause could lie in the fact that females of migratory passerine species sometimes fly longer distances to their wintering grounds, which subjects them to more risks along their route. “They can encounter bad weather, predation, or lack of food,” Wu says. “It’s a really physiologically stressful time during migration.” In particular, the scientists found that long-distance migrants like warblers demonstrated some of the starkest divergences between male and female survival. 

Another reason for the disparity might be baked straight into bird DNA. In mammals, females have two of the same sex chromosomes (XX), while males have different ones (XY). This can help females live longer, because if one X chromosome has a harmful mutation, they have a backup. In birds, however, it’s the males that contain the identical set of sex chromosomes. “Even though most people think that females live longer, because that’s true in a lot of mammal species, it’s not necessarily true across the landscape of life, and this is a really good example of that,” Austad says. 

Before this study, Wu was already an avian feminist of sorts. She previously worked as a scientist for Audubon and now co-leads the aptly named Galbatross Project alongside former colleagues and other like-minded collaborators. The group encourages people to center their birding efforts on female birds—including by spending every Memorial Day weekend looking specifically for them. As the “Galbatrosses” laid out in a paper last year, leaving female birds unnoticed has consequences for science and conservation.  

For instance, because scientists often erect mist nets during breeding season to collect data, they might miss out on capturing  the females that stay at their nests.  Different sexes can also face different habitat threats. In their non-breeding grounds in Central America, for example, male Golden-winged Warblers primarily populate forests at greater elevations, while females inhabit lower altitudes. Because forests closer to sea level offer easier logging, the females have been stripped of double the amount of habitat as the males.  Despite these discrepancies, a 2019 study found that only 8 percent of conservation plans for 66 migratory North American species reference distribution or habitat differences related to sex. 

Such findings having only reinforced Wu’s research focus, as she plans to concentrate the final chapter of her dissertation on the possible sex bias created by habitat use. “I got lucky that this scientific effort does support the cause of the Galbatrosses to draw attention to female birds,” Wu says. “It goes to show that we can’t just assume that what’s true for males is also true for females.”