Female mammals usually outlive males, but a 27-year study found the pattern was reversed among dozens of North American bird species
Female mammals usually live longer than males, but a 27-year study of North American birds has found the opposite pattern in birds.Researchers at the University of California, Los Angeles (UCLA) analyzed more than 563,000 bird-banding records and found that adult female birds had a 12.4% lower survival rate than adult males across 35 bird species. The study, published in Proceedings of the Royal Society B, is the first large-scale research to examine differences in survival between male and female birds.On average, female birds lived for 1.36 years after reaching adulthood, while males lived for 1.61 years. The research was funded by the National Science Foundation.“Female birds are generally studied less than males, so I wanted to focus on them,” said Joanna Wu, a doctoral candidate in ecology and evolutionary biology at UCLA and the study’s lead author. “As I read more studies, I noticed that females often had lower survival rates than males. I expected the opposite because males usually compete for mates and take more risks. These findings surprised me because females are essential for reproduction and keeping bird populations healthy.“
A rare 27-year dataset
Measuring annual survival in wild birds requires catching the same birds multiple times. Scientists usually need between six and ten recaptures for each species to get reliable estimates. To compare males and females separately, they need even more recaptures.Since most wild birds are caught only once during banding, smaller studies often cannot accurately measure survival differences between the sexes.Wu and her team solved this problem by using 27 years of capture-mark-recapture data collected from 1992 to 2018 by the Institute for Bird Populations. The program monitors birds across the United States and Canada by placing small leg bands on captured birds.Working with UCLA researchers Montague Neate-Clegg and Morgan Tingley, along with James Saracco from the Institute for Bird Populations, Wu examined the data alongside physical and ecological traits. These included body weight, clutch size, wing shape, fat reserves, and social behavior.The lower survival rate for females appeared across many different songbird species.“What interested me most was that the same pattern showed up in many kinds of birds, including flycatchers, warblers, cardinals, and tanagers,” Wu said.
The physical demands of migration and reproduction
Every year, birds defend territories, build nests, lay eggs, raise chicks, and search for food. The study suggests that these demands place a greater burden on females.The survival gap was largest in birds that migrate long distances. It was also greater in species with rounded wings and in species where males stored more body fat. Female birds often travel farther during migration, increasing their chances of exhaustion, bad weather, and attacks from predators.Migration is not the only reason. Even female birds that do not migrate had lower survival rates than males, although the difference was smaller.In mammals, females often die more because males compete aggressively for mates and territory. In birds, however, the energy needed to produce eggs, along with the heavier demands of migration, may increase female mortality.
Chromosomes may help explain the difference
Scientists are also studying genetics to understand why female birds tend to die earlier than males.In mammals, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Having two X chromosomes gives females a backup copy if one gene is damaged. Human males do not have this backup and are more likely to develop age-related diseases such as heart disease and cancer as the Y chromosome weakens over time.Birds have the opposite chromosome system. Male birds have two Z chromosomes (ZZ), while females have one Z and one W chromosome (ZW).“In birds, unlike mammals, females carry two different chromosomes while males carry two matching ones,” Wu said. “This could be more evidence that sex chromosomes are linked to shorter lifespans in female birds, but we have not tested that idea directly.”
Climate change and population declines
Scientists are already tracking major declines in bird populations across North America. Habitat loss and climate change are making these declines worse, and researchers want to know whether these environmental changes affect females more than males.If female birds lay eggs and successfully raise their chicks before they die, their shorter lifespans may have little effect on overall population numbers. But if many females die before breeding successfully, the survival gap could speed up population declines.“If females have already reproduced successfully before they die, their shorter lives may not matter much,” Wu said. “But we don’t know if that’s happening. We already know that habitat loss and climate change are the biggest threats, and we now want to understand whether those changes make survival even harder for females than for males.”
