Female birds have shorter lifespans than male birds, researchers find

Birds have a lot to do in their short lives: defend territories, build nests, raise chicks and feed themselves. But the toll of these activities may be greater for female birds than males. A new study by UCLA ornithologists has found that female birds have lower survival rates than males. The research is the first to study female bird survival in a large dataset and shows that the phenomenon is more widespread than previously recognized. In mammals, females generally have higher survival rates, and why birds, which are not mammals, show the inverse is a puzzle the researchers are trying to solve.

"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."

Wu decided to look at female survival in a big dataset to find out how common a shorter lifespan was. Wu and co-authors analyzed more than half a million records of banded birds in North America over 27 years and found that adult survival was, on average, 12.4% lower for females than males in 35 species. Females lived an average of 1.36 years after reaching adulthood while males lived 1.61 years. The paper documenting Wu and the team's work was just published in Proceedings of the Royal Society B. It is the first study to examine avian sex-specific survival rates at a large scale.

"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.

Few large-scale studies of bird survival rates have been done because they require 6-10 recaptures per species to obtain reasonably accurate annual survival estimates. To calculate survival rates for both sexes requires roughly doubling the sample of recaptured individuals to ensure an adequate sample of males and females. But most birds are only caught once, at the time of banding, and never again. Thus, an extremely large dataset is needed to obtain meaningful data.

Wu and UCLA co-authors Montague Neate-Clegg and Morgan Tingley found this in nearly three decades of capture-mark-recapture bird banding data collected by the Institute for Bird Populations across the USA and Canada from 1992-2018. Capture-mark-recapture is a way that biologists estimate the size of an animal population by catching animals, marking them - in this case with a band around the leg - then, after a time, capturing another group of the animals and seeing how many of them are already marked. While temporarily held, the researchers record a variety of other data, which makes this method useful for different kinds of studies. The UCLA group collaborated with co-author James Saracco at the Institute for Bird Populations to sort and analyze the information in relation to factors believed to affect survival, such as migration tendency, clutch size, body mass and social dominance.

Why would female birds live shorter lives than males?

Survival past adulthood is connected to energy and other biological costs associated with reproduction, movement and competition for resources. It takes a lot of energy to migrate and migration also exposes individuals to risk of predation and injury. When they're not migrating, individual birds may exert social dominance to exclude others from prime resources. Producing and laying eggs is a biological cost borne uniquely by female birds, while the work of caring for young might be shared between the parents or not.

The researchers found greater male survival in both migratory and non-migratory birds, although it was strongest in long distance migrants. The effect was also stronger in species with less pointy wings, and where the males had more body fat. But in spite of these trends, there was no one clear explanation for the sex biased survival rates. In general, the findings supported the idea that migration takes a heavier toll on females, who tend to migrate farther than males, but it wasn't clear why female non-migratory birds also tended to have shorter lifespans, though the difference was smaller. In many mammal species, heavy movement and social dominance costs contribute to lower survival rates for males in adulthood, but the case is not exactly the same in birds.

One possibility that could be explored in future research, according to Wu, has to do with chromosomes. In mammals, females carry two copies of the same chromosome (XX), while males carry two different chromosomes (XY), one of which (the Y) influences the sex. This means that if there is a faulty gene on one chromosome, females might have a backup on the other one. But males only have one copy of each chromosome, so faulty genes are more likely to be expressed. In human males, this lack of chromosomal backup is associated with higher susceptibility to heart disease, cancer and other problems as age causes the Y chromosome to degenerate. Thus, many researchers believe there is a connection between longevity and whether a pair of chromosomes is the same or different. But in birds, the chromosome distribution is reversed.

"In birds, unlike mammals, it is the females who carry two different chromosomes, while males carry two of the same," said Wu. "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."

Future research could also explore whether the shorter lifespan of females contributes to the dramatic decline of bird populations scientists have noted.

"If the females have already successfully reproduced before they die, their shorter lives might not make much of a difference," said Wu. "But we don't know yet if that's the case or not. 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."

The research was funded by the National Science Foundation.