LOS ANGELES — Female birds in North America have significantly shorter adult lifespans than males, according to a new UCLA study published July 30 that analyzed more than half a million banding records spanning 27 years and found survival rates about 12% lower for females across 35 species.
The finding, the first large-scale examination of sex-specific survival in birds, challenges expectations drawn from mammals—where females typically outlive males—and raises questions about whether the higher costs of migration, reproduction, or even sex chromosomes help explain why female birds die younger. With many bird populations already declining due to habitat loss and climate change, researchers say understanding these sex differences could matter for conservation.
“Female birds, in general, are understudied relative to males, so that’s what I wanted to research,” UCLA doctoral candidate in ecology and evolutionary biology Joanna Wu said. “As I began reading papers on female birds, I was surprised to notice a pattern that the survival rate seemed to be lower in females than males. I expected that males would have lower survival because they’re usually the ones competing for mates and so forth. So these previous findings concerned me, since females are crucial to reproduction and the continuation of populations over time.”
The UCLA researchers drew on data from the Monitoring Avian Productivity and Survivorship (MAPS) program, a continent-wide network coordinated by the Institute for Bird Populations. Since its start in 1989, more than 1,400 banding stations across nearly every U.S. state and Canadian province have contributed almost 2.5 million capture records—making it one of the largest long-term demographic datasets for North American landbirds.
Researchers catch birds in fine-mesh mist nets that are nearly invisible when stretched between poles in the woods. Trained banders carefully untangle each bird, slip a lightweight numbered aluminum band onto one leg, record its species, age, sex, and measurements, then release it unharmed—usually within minutes. The hard part is getting the bird back: most individuals are never seen again. They may disperse, migrate hundreds of miles, die, or simply avoid the nets in future years. In the UCLA study’s massive dataset of more than 563,000 adult birds, only a small fraction were recaptured, which is why such enormous sample sizes are required to estimate survival rates with any confidence.
North America has lost nearly 3 billion birds—about 29% of its total breeding population—since 1970, according to a landmark 2019 study published in the journal Science. Led by Kenneth Rosenberg of the Cornell Lab of Ornithology and American Bird Conservancy, the analysis combined decades of data from multiple long-term surveys (including the North American Breeding Bird Survey) and weather-radar estimates of migratory biomass. The losses hit both rare and once-common species across almost every habitat, with grassland birds declining by more than 50%.
In many bird species, population growth is limited by the number of breeding females—the sex that produces the eggs. When adult females already survive at lower rates than males, as the UCLA study found across dozens of species, further losses of females can leave a surplus of unpaired males and fewer successful nests. Demographers note that this sex-biased mortality can accelerate declines beyond what overall population counts alone would suggest, especially in already-stressed species facing habitat loss or climate pressures.
Male and female birds often experience different environmental pressures because they frequently use separate wintering grounds, migrate different distances, or occupy distinct habitats. In many songbirds, for example, females tend to winter farther south or in more marginal habitats than males. As climate change alters temperatures, storm patterns, and food availability—and as habitat loss accelerates—these differences can expose the sexes unequally. A drought or heat wave that hits one wintering region harder than another, or the conversion of forests preferred by females, could therefore widen the survival gap already documented in the UCLA study.
