Why do parent birds throw their young out of the nest?

Yellow throats

Young bird of the willow yellow throat marked with a sensor. (Image: Todd Jones / University of Illinois)

Many songbird parents throw their not yet flyable nestlings out of the nest. Researchers have now investigated why. Accordingly, this behavior reduces the chances of survival of individual young birds. However, this reduces the risk that the entire brood, including parents, will become victims of predators. In potentially dangerous environments in particular, these bird species increase their evolutionary fitness if they invest in a few, but independent, young birds.

Animals instinctively pursue the goal of passing on their genes to the next generation. Therefore, they often invest a lot of energy in raising their offspring in order to ensure their survival. Nevertheless, researchers repeatedly observe behaviors in parents and their offspring that do not seem evolutionary at first glance. For example, many songbird parents throw their young out of the nest before their plumage is fully developed and they can fly. As a result, the young birds are at the mercy of predators and have to look for shelter on the ground or in branches close to the ground.

Songbird parents under observation

If the young birds are not yet able to look after themselves and are in danger of dying, why do their parents throw them out of the nest? Scientists working with Todd Jones from the University of Illinois have now researched this. To do this, they examined the behavior of the parents and their young in 18 different songbird species that were breeding at eight different locations in the US states of Florida, Illinois, Ohio, Pennsylvania and Texas. They compared the survival rates of individual juveniles in the nest and after they fledged. On the basis of this, they also determined the probability of death for the entire brood before and after the ability to fly.

The result: Twelve of the 18 bird species threw their offspring out of the nest early. This reduced the chances of survival for these young birds by almost 14 percent. Because outside of the nest, the young birds were exposed to their environment independently and had to protect themselves from predators such as birds of prey or snakes – without being able to fly. Only when the young fledged within the next five days did their chances of survival increase again.

Discharge increases the fitness of the parents

But why do so many of the parent birds risk their young to die? “Parents spread the risk,” explains Jones colleague Mike Ward. “The longer the young birds stay in the nest, the greater the chance that the entire brood will be lost to predators such as snakes or raccoons.” So instead of looking after all the young in the nest together, the bird parents separate them by removing some of them prematurely Throw nest. “That way, the chances of all of them dying are almost zero,” continued Ward. This behavior is especially beneficial for the parent birds who have large broods and breed in risky environments.

“Individual fledglings may be less likely to survive,” explains Jones. “But if you throw them out of the nest early, the parents benefit from a 14 percent higher probability of raising at least one offspring to become self-employed.” According to this, it seems to make more sense for the bird parents in risky environments to keep a few, but as survivable as possible To have offspring to successfully pass on their genes. “Our results suggest that parents treat their offspring in such a way that they can fledge earlier and to improve their own fitness – even at the cost of survival for each of their offspring,” the researchers sum up.

Compromise parental care

The behavior of the parents is therefore a compromise of parental care, explain Jones and his colleagues: If the parents invest a lot of energy in rearing, this promotes the survival of the individual boys. On the other hand, the parents exhaust themselves through the great effort and are more exposed to predators or diseases and can also produce fewer additional offspring. On the other hand, too little care reduces the chances of survival of the offspring, so that the parents’ genes do not pass into the next generation.

Although ejecting the nest primarily improves the fitness of the parents, the surviving offspring also benefit from it, according to the researchers: “While the offspring have to expend a lot of energy at first, it is also beneficial for them if they breed in the future and do the same with theirs Make boys. The behavior is passed on from generation to generation, ”explains Jones.

Source: University of Illinois, Article: Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2008955117

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