(CN) — Many songbird parents give nestlings the boot well before they're ready. But it’s not bird-child abuse — it’s for a greater genetic good.
Young birds that stay in the nest longer increase their chances of survival. But continuing care may impose an overall cost on parents by decreasing the fitness of the entire brood. The influence of this kind of parent-offspring conflict on the age at which songbirds fledge, or leave the nest, is unclear. Todd Jones, doctoral student in the Department of Natural Resources and Environmental Sciences (NRES) at the University of Illinois, and colleagues analyzed data from eight studies of 18 songbird species across the United States.
The study, published in the Proceedings of the National Academy of Sciences, shows the behavior appears in 12 of 18 species in various habitats throughout the country. Scientists found that while individual chicks might be less likely to survive under these circumstances, the chances of raising at least one offspring to independence go up by 14%.
"The parents are distributing the risk," says Mike Ward, associate professor in NRES and senior author on the study. "The longer chicks stay in the nest, the greater the chance the whole brood will be lost to predators like snakes or raccoons. But we see parents physically separating chicks in space outside the nest, and that way, the probability of them all dying is almost zero."
Aspects of the study touch on the concept of parent-offspring conflict, the tradeoffs inherent to parental care. Putting a lot of resources into kids favors their survival, but can leave parents depleted, at risk of predation or disease, and potentially less able to produce additional offspring. Too little care, and offspring may not survive to carry parents' genes into the next generation. And from an evolutionary perspective, successfully passing genes along is the number one goal.
"For all organisms with parental care, there will always come a point where they're in conflict. In this case, it was a little surprising parents were putting chicks out in a dangerous situation that's good for the parent, but not good for the offspring," Ward said. "But it makes sense from an evolutionary perspective, for the parents."
The study is the first comparing survival rates before and after fledging across species and locations, demonstrating an almost universal post-fledging survival decline in these songbirds. It also establishes a baseline for what to expect, survival-wise, against a backdrop of environmental change.
"Some of these species are declining pretty dramatically throughout the Midwest," Ward noted. "They're probably right on the razor's edge. So if predation goes up for some reason, it could really have big implications for that cohort throughout the years."
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