An analysis of more than 563,000 banding records found adult female survival was 12.4 percent lower than that of males, with the gap widest among long-distance migrants.

In many mammals, including humans, females tend to outlive males. Birds, it turns out, may often follow the opposite pattern. A study led by researchers at the University of California, Los Angeles, has found that female songbirds in North America survive at lower rates than males across a wide range of species.

The work, published in Proceedings of the Royal Society B, analyzed more than 563,000 records of birds captured, banded and released between 1992 and 2018. Across 35 species, adult females had a survival rate 12.4 percent lower than males. On average, females lived 1.36 years after reaching adulthood, compared with 1.61 years for males.

The project began with a gap in the research. “Female birds, in general, are understudied relative to males, so that’s what I wanted to research,” said Joanna Wu, a UCLA doctoral candidate in ecology and evolutionary biology who led the study. “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.”

That was not what she had expected. “I expected that males would have lower survival because they’re usually the ones competing for mates and so forth,” Wu said. “So these previous findings concerned me, since females are crucial to reproduction and the continuation of populations over time.”

Decades of banding data

The data came from a network of bird-banding stations across the United States and Canada coordinated by the Institute for Bird Populations, a California-based nonprofit. At such stations, trained volunteers and biologists catch birds in fine mist nets, fit each with a small numbered leg band and record its species, sex, age and condition before releasing it. When the same bird is caught again in later years, researchers learn that it survived.

Wu worked with UCLA colleagues Montague Neate-Clegg and Morgan Tingley and with James Saracco of the Institute for Bird Populations. Estimating survival from such records is not simple, because a bird that is not recaptured may have died, moved elsewhere or simply avoided the nets. Statistical models account for these possibilities, but they need enough data to work. The team found that reliable survival estimates required between six and ten recaptures of a species.

The pattern held across many kinds of birds. “What was interesting to me was that this pattern was consistent across many different types of birds, from flycatchers and warblers to cardinals and tanagers,” Wu said.

Migration takes a toll

The male survival advantage was strongest in long-distance migrants, birds that travel between breeding grounds in North America and wintering areas far to the south, sometimes in Central or South America.

The study does not pin down why. Migration is among the most dangerous periods in a bird’s life, with risks from storms, exhaustion, predators and collisions with buildings. Females may face particular costs. Producing eggs demands large amounts of energy and nutrients, and in many species females do most or all of the incubation, leaving them with less time to recover before the long journey south. Differences in where males and females spend the winter, or in how they compete for food and territory there, could also play a role.

Earlier research has also shown that the sexes can face different risks at different times of year. In some species, males return to breeding grounds earlier to claim territories, exposing them to late cold snaps, while females may migrate on different schedules or to different wintering areas, where habitat quality can vary considerably.

A chromosomal clue

Wu also pointed to a possible genetic factor. In mammals, females carry two X chromosomes and males carry an X and a Y. One hypothesis for why female mammals often live longer is that having two copies of the X chromosome provides a backup if one copy carries a harmful mutation. Birds have a different system, known as ZW.

“In birds, unlike mammals, it is the females who carry two different chromosomes, while males carry two of the same,” Wu said. “This could be further evidence connecting the sex chromosome to reduced longevity in female birds, but that’s a question we haven’t directly investigated yet.”

That idea, sometimes called the unguarded sex chromosome hypothesis, has gained support from studies comparing lifespans across many animal groups, but it remains debated, and the new study was not designed to test it.

What it means for bird populations

The finding matters because females set the pace of population growth. In general, a population’s ability to produce the next generation depends on how many females survive to breed. North America has lost an estimated three billion birds since 1970, and lower female survival could make some populations more vulnerable than overall counts suggest.

Whether that is the case depends on timing. “If the females have already successfully reproduced before they die, their shorter lives might not make much of a difference,” Wu said. “But we don’t know yet if that’s the case or not.”

The researchers want to understand how human-driven environmental change affects the gap. “We know that habitat loss and climate change are the main culprits, but we want to learn if those factors might be making it even harder to survive for females versus males in a world that humans are rapidly changing,” Wu said.

The study has limits. It covered species that are commonly caught at banding stations, mostly small songbirds, so the results may not apply to birds such as seabirds, raptors or waterfowl. Banding data also cannot reveal where or how birds die. Tracking technologies, such as tiny GPS tags and automated radio-telemetry networks, could help answer those questions in the future.

For now, the results suggest that conservation planning may need to pay closer attention to the sex of the birds being counted. A stable number of birds at a feeder or in a survey may hide a population quietly losing its females.

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Notes

1. Wu, J., et al. Study on sex differences in survival of North American birds. Proceedings of the Royal Society B, 2026. DOI: 10.1098/rspb.2025.3227.
2. University of California, Los Angeles. “Female birds have shorter lifespans than male birds, researchers find.” EurekAlert!, 30 July 2026. https://www.eurekalert.org/news-releases/1138278.

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