When is the best time to leave the nest? That’s the dilemma encountered by birds the world over.
Staying home longer helps juvenile birds grow their wings and improve their chances of fleeing from predators, but each day that nesting continues increases the risk of the whole brood being eaten.
A team of researchers from the University of Montana in the US used a variety of approaches to take a closer look at this previously unrecognised conflict between the needs of young birds and their parents. The team, led by Thomas Martin, analysed long-term data on nest predation and nestling growth for 19 songbird species local to the state, and examined the flight performance and fledging ages of 11 of these species using high speed videography obtained using a Go-Pro camera.
The researchers found that in species with younger fledging ages, flight performance in the young birds was poorer and predation rates were high.
In order to test their results experimentally, Martin and his colleagues erected enclosures around the nests of grey-headed juncos (Juncos hyemalis). The enclosures allowed the parents to come and go to feed the young, but when the young fledged naturally, they were protected from predators for a further three days.
The survival of enclosed young was substantially higher than those who had fledged at the normal age in unprotected nests. The mass of the young birds in both groups was similar, but the wing length much longer in the enclosed group.
So why aren’t selection pressures for songbirds favouring a later nest departure? Earlier studies show that each day that the nest is occupied substantially increases the risk of nest predation. When nestlings leave the nest earlier than they would like, at least some survive. The survival of some of the young, rather than the loss of the entire brood, benefits the parents and their investment in breeding.
Songbird parents try to coax their young out of the nest by offering food in an effort to get them to leave. And this pressure goes both ways, as the nestlings exert considerable control over the parents with their begging behaviour in the form of incessant cries.
The team found that this push-pull of different generations is “yielding a compromise between parents and offspring that balances risk of mortality in versus out of the nest”.