Why female songbirds live shorter lives than males

Among humans and most other mammals, females generally outlive males. Birds show the reverse trend: across dozens of North American species, females die earlier than males.

Joanna Wu, a doctoral candidate at the University of California, Los Angeles (UCLA), became concerned after noticing this disparity while reviewing older research papers.

“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,” said Wu.

She set out to establish whether the trend would remain in a larger dataset. Along with three colleagues, she examined one of the continent’s biggest collections of bird data.

Female songbirds died younger

Wu and her colleagues compiled more than half a million records for ringed birds, spanning 92 species over 27 years.

These records were supplied by a nationwide scheme operated by the Institute for Bird Populations (IBP).

Females had lower survival rates than males in 82 of the species.

The gap was sufficiently large to achieve statistical confidence in 35 species. The same pattern was also seen in eight of the nine bird families with enough data for comparison.

On average, 48 percent of adult females survived from one year to the next, compared with 54 percent of adult males.

In other words, females had a 12.4 percent lower survival rate, while males lived roughly 18.5 percent longer once they had reached adulthood.

“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,” said Wu.

Nearly three decades of ringed birds

The information came from the long-established Monitoring Avian Productivity and Survivorship programme.

Since the late 1980s, volunteers and biologists have taken part at more than 1,200 locations throughout the United States and Canada.

Bird ringers capture birds using fine nets, attach a numbered ring to a leg, note the bird’s sex and release it. Some of those birds are encountered again years later.

By measuring these returns, biologists can estimate the typical lifespan of each sex. However, the method has a limitation: a female that is not recaptured may have died or permanently moved elsewhere.

James Saracco, a biologist working with the programme, arranged the records according to traits believed to influence survival. These included the distance a species migrates and how much fat its birds accumulate.

Migration takes a bigger toll

Wu’s team then explored possible explanations for the disparity. The greatest differences appeared among long-distance migrants, which travel furthest between their breeding areas and wintering grounds.

Female songbirds frequently migrate farther than males from the same species. Those longer journeys bring increased risks, including predators, storms and exhaustion.

Wing shape also played a part. Species with more rounded, less pointed wings-which are less efficient for extended flights-had a larger survival difference between females and males.

Body fat provided another indication. In species where breeding males had more fat than females, males had a greater survival advantage, suggesting that they may be more successful in competition for food and territory.

Egg-laying was not the answer

Many people might assume that laying eggs takes a heavy toll on female songbirds. Wu’s team examined this possibility but found no connection.

Species with larger clutches, or those investing more energy in every egg, did not have wider survival gaps. Egg-laying therefore does not appear to explain females’ shorter lives.

This supports previous research. Scientists studying 194 bird species likewise identified no association between reproduction and increased female mortality.

The finding runs counter to common expectations, because only females lay and incubate eggs.

One interpretation is that feeding and protecting chicks-a responsibility usually shared by both parents-may demand more than producing the eggs initially.

A sex chromosome reversed from ours

The researchers could not test one possible explanation involving sex chromosomes. In many animals, the sex with two different chromosomes dies younger because a harmful gene on one chromosome lacks a healthy counterpart.

For mammals, this is males, which carry an X and a Y chromosome. Birds have the opposite arrangement, with females carrying the two different chromosomes.

“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.”

What shorter lives could mean

There was one exception among the families studied. Woodpeckers-the sole non-songbirds in the sample-had no survival difference at all between females and males.

Wu next hopes to determine whether females’ shorter lifespans are contributing to the wider decline in North American bird populations.

“If the females have already successfully reproduced before they die, their shorter lives might not make much of a difference,” noted 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.”