Photograph by Vickie Anderson, Nat Geo Image Collection
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A dark-eyed junco fledgling sits on a mound of mud.

Photograph by Vickie Anderson, Nat Geo Image Collection

Birds That Leave Nest Too Late Can Endanger Their Families

A baby bird’s ability to move and its chances of survival are closely tied, a new study finds.

Humans and birds have a surprising amount in common. Both can accomplish seemingly complex tasks, like distinguishing between faces and expressions and making musical instruments. And when it comes to raising their young, deciding when to move the kids out is another thing bipeds and avians have in common.

In both human and bird scenarios, parents often encourage their offspring to leave the nest earlier rather than later. For humans, the decision is nuanced, but for birds, their lives could depend on it.

In a recent study published in Science Advances, University of Montana researchers looked at how the age when birds leave the nest affects their survival. Turns out, survival of birds is closely tied to how ready they are to fly. Younger birds are more vulnerable and less likely to survive in the wild than older ones, but older birds could crowd the nest and attract predators. (Related: “These Birds Decorate Their Nests With Trash—Here’s Why.”)

Time to Fly

Tom Martin, the paper’s corresponding author, is the assistant unit leader and research scientist of the Montana Cooperative Wildlife Research Unit. His past research on birds, domestic and international, sparked the current study.

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The paper looked at about a dozen songbird species, including the American robin, red-faced warbler, and other arboreal species, as well as ground-dwelling birds like the house wren and white-breasted nuthatch. Martin says the researchers chose these birds because they have a wide variety of wingspans and leave the nest at different points in their lives. (Related: "This Bird Has Been Singing the Same Song for 1,000 Years")

“I wasn’t surprised by the general results,” Martin says. “The experiment was the confirmation. You’re always a little uncertain until you do an experiment.”

As expected, younger birds had poorly developed wings and were not-so-great flyers. This makes them more vulnerable to predators, so if they left the nest before they can fully develop, their survival rate dips. The death toll for early birds can be as high as 70 percent. (Related: “Why Do Small Birds Have Sweeter Songs?”)

But, the death toll hovers around 12 percent for late bloomers. Birds that leave the nest later were more developed and better flyers that were more likely to survive in the wild. When the researchers kept some birds, like the gray-headed junco, in the nest for a few extra days, their chances of survival increased.

Although more time in the nest could save a young chick’s life, it could spell disaster for the rest of its family. Most bird parents encourage their young to leave the nest early because more chicks mean more noise, which can attract hungry predators. (Related: “5 Baby Bird Cams You Don’t Want to Miss This Spring”)

“If the young get discovered by a predator when you’re in the nest, then all the young get eaten,” Martin says. “The young want to stay and the parents want them out.”

The parent birds have to negotiate with the fledglings when to leave the nest. Birds that nest on or near the ground, like the house wren, push their young to leave early. Moving on the ground is more dangerous than moving in trees, so house wrens have higher chances of mortality.

Long-Term Research

Studies in the past have looked at animals’ locomotive performance, or their ability to move, and at their survival rates. But few have looked at both these factors together in specific species, and none have examined the combination in birds.

“The consequences for survival of the young have not been studied in the wild,” Martin says.

Although the results in this study were not surprising, they’re helpful for future research. Martin studies birds in the tropics, so he says another project could look at locomotion and survival rates in those species.

“This was looking at the short-term effects,” Martin adds. “The consequences for the young over a longer time period need to be examined.”