It's a struggle many human parents can relate to – you've given the kids a good start in life and now you'd like them to leave home so you can have your old life back. Trouble is, they don't want to go, leading to a battle of wills familiar enough to be the stuff of film. Songbirds have the same conundrum and a new study shows some species risk the lives of the offspring they worked so hard to raise. Moreover, they've given parents accused of being selfish the perfect comeback – it's for the good of the species.
Unlike most fish or octopuses, birds lay only a few eggs and invest heavily in their survival to maturity, but there is always a limit. Dr Mike Ward of the University of Illinois College of Agricultural Consumer and Environmental Sciences decided the key to whether birds are being forced out prematurely lies in how many fledglings die soon after leaving the nest.
In Proceedings of the National Academy of Sciences, Ward and co-authors note that zoologists have long debated whether songbird “parents manipulate offspring into fledging to optimize their own fitness or do offspring choose when to leave?”
Out of the 18 species of North American songbirds studied, 12 experience sharp spikes in mortality in the first few days after leaving the nest. Based on past research the authors concluded that survival prospects for individual birds would be better if they stayed in the nest longer, suggesting the parents are booting them out before they are ready to go.
The reason wasn't hard to find. “For all organisms with parental care, there will always come a point where they're in conflict,” Ward said in a statement.
On average, birds were 13.6 percent less likely to reach maturity when forced from the nest early, but the parents were an almost identical 14 percent more likely to raise at least one more child when they pushed the previous brood out prematurely. For the species as a whole, risking the eldest offspring can be a wise choice if it makes room for more. A secondary benefit is making sure that at least one member of a brood survives. For birds raising many young at once, losing all of them to a single predator attack is a disaster; once the young have been pushed out, they go their separate ways and a predator is unlikely to get all of them.
Nevertheless, these averages hide wide variations between species. Brown-headed cowbirds suffer the biggest rise in mortality when they leave the nest, but it's not surprising their welcome is short considering they are brood parasites raised because their biological parents used mafia tactics to make others foster them.
However, cerulean warblers’ daily survival rates also plunge dramatically post-fledging, whereas the increase in mortality for northern cardinals is barely detectible. Six of the studied songbirds species allow their young to stay until leaving is safe. Indeed, daily survival for the golden-cheeked warbler actually improves marginally outside the nest.
"Some of these species are declining pretty dramatically throughout the Midwest. 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,” Ward said.
Sadly for anyone seeking tips, the researchers did not observe how parents got their young to leave, nor if any young have developed tactics to extend their stay.