Science

Why Some Songbirds Kick Their Chicks Out Before They Can Fly

A new study finds that nest type plays a big role in how long young birds can live at home.

Every summer, teenage humans and birds alike leave their parents’ nests to face the big, wide world on their own. For many an angsty high school graduate, the chance to flit away to begin independent lives often feels long overdue. Their avian counterparts, however, aren't always as ready to leave.

In the nest, young birds have it easy: free food delivered straight to their open mouths, fueling their growth. But the bigger and louder they get, and the more time that passes, the greater the risk that a predator will discover the nest. To avoid losing their entire brood, songbird parents try to hustle their adolescents along, eventually forcing them from the nest. Some species will even go so far as to stop feeding their chicks in the nest, instead using food to lure them out—sometimes even before they can fly. 

With parent and offspring pitted against each other, many species have evolved a compromise to decide when it's time for young songbirds to leave home, balancing the chicks' individual needs with the family's safety, according to a study published this week in Science Advances. Long-term data and experiments show that how many days a chick stays in the nest is linked to whether the species nests in an exposed cup or inside a concealed cavity. 

“It’s something most people can relate to, the idea of a parent-offspring conflict,” says Thomas Martin, lead author on the new research, who has dual appointments at the University of Montana and U.S. Geological Survey. “The offspring want to stay in the nest to increase individual survival, whereas the parents want to get all out of the nest to determine that at least some of the young lives.”

Martin and his colleagues studied 19 different songbird species for 35 years, monitoring nesting strategies, vulnerability to predators, and wing development. Then, to determine how well 11 of those species could fly when they left the nest, he brought in Bret Tobalske, an expert in flight biomechanics at the University of Montana, who performed drop tests, which involve dropping a baby bird and a golf ball onto a soft pillow to see which is better at resisting gravity.

The results showed that birds that leave the nest at a younger age—like Gray-headed Juncos (one of several subspecies of Dark-eyed Junco), which stay for only nine or 10 days—have less developed wings, and as a result often die before adulthood. Those that stay home longer—like Mountain Chickadees, which typically stay in the nest for 22 days—can fly away when they finally leave, giving them a better chance of survival. 

So why would parents ever let their young leave before they can fly? The difference is how easy it is for predators to find or access the nest. Species that use cavities, like the chickadees, are safer from predators than species that use open cups, like the juncos. When snakes, jays, or squirrels find bird nests, more often than not they’ll destroy the entire brood, Martin says. So for the adult juncos, even if only one of their offspring survives their early days of independence, it's worth the tradeoff.

“I think [this study]’s done a really good job of formalizing a concept that, at least in my mind, most ornithologists already knew,” says Bridgit Stutchbury, a conservation biologist at York University who was not involved with the study. “As I was reading it I already knew it, but no one had proven it with really in-depth science.”

When people see a baby bird on the ground, they may try to rescue it, Stutchbury says. This new study underscores common birder wisdom: “Just leave it alone,” she says. “It’s fine and the parents probably aren’t far away.” Birds like juncos will continue to feed their fledglings for a few days or even weeks after they leave the nest to ease their transition to full independence. Other parents are “second-nesters,” rushing through raising their first brood so they can have a second before the summer’s out. In some species, like American Robins, the male will feed the fledglings while the female gets started on the second nest; in others, the adolescents are left to make their own way.

Both strategies have the same goal: get as many chicks to survive to adulthood as possible.

For the young birds that want to stick around, being forced out of their nest might seem like a harsh parental tactic, but that just means the evolutionary compromise has kicked in. With no other options, the little birds have to seek shelter and begin making their own way in the world, with the hope that one day they'll get to give their own chicks the boot. 

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