Why Some Songbirds Coax Their Young Out of the Nest Early

They hope it will help their long-term chances of survival.

Radio-tagged fledgling Common Yellowthroat
Radio-tagged fledgling common yellowthroat.

Todd Jones

At some point, all baby birds have to leave the nest. But songbirds often evict their young long before it’s actually time for them to spread their wings and fly, a new study finds.

Research from the University of Illinois finds that many bird parents kick out their nestlings early for mostly selfless reasons.

Of the 18 songbird species they studied, researchers found that 12 of them encouraged their offspring to leave their nests early.

“From what we can tell, they don’t physically push them out, but manipulate them into leaving the nest with food or through hunger,” lead author Todd Jones, doctoral student in the Department of Natural Resources and Environmental Sciences at University of Illinois, tells Treehugger.

The young birds who were coaxed to leave early were about 14% less likely to survive than those that stayed in the nest. 

Then why would songbirds push out their babies before they are ready?

“Parents do this to reduce the odds of losing the entire brood to predation. In other words, parents avoid leaving all their eggs (or in this case nestlings) in one basket,” Jones says.

By encouraging their young to fledge earlier, they can physically separate them and lower their chances of losing all of them to predators like snakes and weasels to nearly zero.

“In contrast, if offspring were to remain in the nest for longer, parents would be more likely to lose the entire brood, as when a nest is predated the entire brood is usually lost,” Jones says.

The findings were published in the journal Proceedings of the National Academy of Sciences.

A Learned Behavior

Researchers believe that the surviving birds likely learn from their parents and repeat the behaviors with their own nestlings.

“While individual offspring suffer in the immediate term, later on in life when those individuals are breeding, they do the same thing to their own offspring, and therefore benefit from the behavior,” Jones says. “Our study suggests this strategy ultimately improves parental fitness and is likely passed on genetically from generation to generation.”  

Songbirds aren’t the only animals that encourage their babies to leave home prematurely. In the bird world, raptors and seabirds also do this by limiting the amount of food they give their young in order to ease them out of the nest.

“For animals with parental care there is eventually conflict between parents and offspring over when care will end. This doesn’t always result in costs for offspring, as in the case of our study, but in many cases it can and it can be quite extreme,” Jones says.

Some solitary big cats will drive away their young so they can reproduce again. Many fish and beetles will kill or eat their young for their own survival or to improve the survival odds for their remaining offspring.

“Our study improves our understanding of parent-offspring conflict, a concept in evolution describing tradeoffs between offspring care and survival in parents, which is responsible for many of the behaviors we see throughout the animal kingdom, including in humans,” Jones says.

This is the first study, researchers say, that compares survival rates before and after fledging across many species and locations, demonstrating a near-comprehensive post-fledging survival decline in the songbirds studied. It also provides a baseline for what strategies birds might use to respond to environmental change.

Jones says, “Birds face many challenges in our rapidly changing world, and it is critical that we understand strategies, such as the one documented in our study, that birds may use to respond to such challenges so that we may conserve these avian species.”

View Article Sources
  1. Jones, Todd M. et al. "Parental Benefits And Offspring Costs Reflect Parent–Offspring Conflict Over The Age Of Fledging Among Songbirds". Proceedings Of The National Academy Of Sciences, 2020. doi:10.1073/pnas.2008955117.