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A junco parent lures a youngster from the nest, despite its poorly developed wings.

T. E. Martin

Time to fly? Birds that leave the nest at the wrong time can bring disaster on the whole family

For parents with grown children, sometimes the pain of an empty nest can be better than the pain of a full basement. It turns out birds face a similar, but slightly more deadly, dilemma. When young birds leave the nest early, they help future generations survive better, but they themselves are more likely to die, according to a new study.

“There’s no single optimal solution,” says Rick Relyea, an ecologist at Rensselaer Polytechnic Institute in Troy, New York, who was not involved with the study. “This work is not only about birds,” he adds, “it’s about how all animals evolve to make this decision.”

Young birds can have a tough life—as can their parents. A noisy nest attracts predators that can wipe out the entire year’s reproductive efforts in one fell swoop. Thus, avian parents push their young to leave the nest early—even when they are not quite ready—increasing the chances that at least one will survive; literally making sure all their “eggs” are not in one basket. But these early birds suffer as a result: Their death toll can be as high as 70%, compared with just 12% for species that are late bloomers.

To understand the reasons behind that difference, the ecologist who first discovered it—Thomas Martin of the U.S. Geological Survey in Missoula, Montana—teamed up with biomechanists Kenneth Dial and Bret Tobalske at the University of Montana in Missoula. Suspecting survival rates had something to do with how ready the birds were to take wing, they tested fledgling flying ability at different ages in about a dozen species and recorded the results with high-speed video.

As expected, younger birds had poorly developed wings and flew badly, if at all. Moreover, when researchers forced a grayish sparrow called a junco (which has some of the lowest fledgling survival rates among the species studied) to stay in the nest for 13 days instead of 10, more lived to adulthood. Just 10% perished within 7 days, compared with 30% for the early birds, the team reports today in Science Advances.

But it turns out that some avian moms are willing to keep their kids in the nest a bit longer. Birds such as the white-breasted nuthatch, which nest in well-protected tree cavities, let the young linger longer at home, Martin’s crew discovered. In contrast, birds that nest on the ground or in the open pushed for early departures. That’s because they suffer comparatively high losses of chicks in the nest. “Depending on where you nest—that drives the decision about when to stay and when to go,” Relyea says.

Are there any lessons for human parents? Some parallels exist, Relya says, but with people, “there’s so much more culture that comes into it.” That makes the decision a whole lot more complicated.