Willow Warbler. Photo: Andrew Howe/iStock

Science

The Rise of the Patriarchy Is Bad News for Birds (Duh)

Males are dominating smaller groups of migratory birds—a phenomenon that could accelerate local extinctions.

It seems humans aren’t the only ones tweeting about smashing the patriarchy. A study published last week in the Journal of Animal Ecology found that female birds are leaving their male counterparts behind, but not for the reasons you’d expect.

By surveying 8,000 Willow Warblers from 34 sites over 18 years, scientists from the University of East Anglia found troubling population trends that seem to be tipping the balance between females and males. To cope with increasingly fragmented habitats, female birds in south and east England are flying to larger pastures that boast better living conditions and a more balanced ratio between the sexes. This, in turn, is leaving some flocks completely lady-less. It’s a tragedy for the jilted males—doomed to live out their lives as childless virgins—but it’s also a blow to the local Willow Warblers, which have suffered a sharp decline of up to 41 percent in some populations.

The gender bias doesn’t end there. The research, which used data from capturing, marking, and recapturing birds, revealed that females have lower survival rates than males. But this discrepancy didn't crop up in the smaller, male-heavy populations, which proves it isn't the driving force behind the testosterone-fueled takeover.

The true catalyst, scientists think, is the birds' breeding habits. In some migratory species, like the Willow Warbler, males will return to mate in the same place they hatched. But females lack this instinct; instead, they’ll scout out the best location for their breeding needs. They look for three important factors: a healthy environment, an abundance of males, and an approximately equal ratio between the sexes. And though Willow Warblers live just two years, they return to the same site each year. “That first year is really important,” lead researcher Jennifer Gill says. “That first decision [by the female] lasts.”

So while females are blessed with the ability to choose, males are stuck with the short end of the stick, waiting for a contingent of mates that may never arrive. “They rely on females flying in from elsewhere,” Gill says. “And if they don’t [show up], these small and vulnerable populations will die out.”

This results in a neverending loop that spans generations: Skewed sex ratios lead to fewer birds reproducing, which leads to fewer females in local populations, which drives the concentration of males up even more. That at least is the case with the Willow Warbler. In 1994, the ratio was close to even in the studied populations; but by 2012, male birds had climbed to 60 percent. Ultimately, the researchers see this as a model for a number of similar species. "Whatever is happening to the Willow Warblers is probably happening to other migratory birds as well,” Gill says. In the worst scenarios, the outcome could be local extinction.

Gill also points out that the study may have underestimated the true extent of the gender gap. The scientists relied on singing males to record the number of male-female breeding pairs; in populations with a healthy sex ratio, one singing male equaled one singing female. But male birds that find themselves without mates will continue to croon throughout the season. These lonely yet vocal males could have been incorrectly counted as a breeding pair.

As agricultural development continues to splinter the warblers' habitat in England, the battle of the sexes will only intensify. Much of their larger tracts of territory have already dissolved into farmland; remaining patches of wetland are being taken over by wheat fields.

“We spend a lot of time trying to maintain little pockets of biodiversity, but this study shows that might not be a good idea,” Gill says. “We now know we need to create connectivity in landscapes, to recreate large areas that are capable of holding bigger populations.” She adds that future conservation efforts should focus on collaborations with local farmers to set aside breeding spaces for Willow Warblers. If more females can be wooed to the region by habitat, the males will get a chance to win them back. Being single forever is no kind of privilege—at least, not in the bird world.

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