July 6, 2012
When Do Youngsters Fly The Nest?
redOrbit Staff & Wire Reports - Your Universe Online
As seabirds mature and reach a time where they can fly the nest, their parents begin to feed them less each day. However, according to researchers from the University of Leeds, it is actually hormones that control when the chicks will leave home.In their study, the researchers wanted to pinpoint the main trigger which causes chicks to leave the nest and get on to an independent life, a process known as fledging.
While studying a colony of Manx Shearwaters (Puffinus puffinus) on the island of Skomer, researchers from the University's Faculty of Biological Sciences noticed that parent birds seemed to become increasingly numb to their chicks' demands for food as fledging approached. At the same time, the chicks showed a marked increase in levels of the hormone corticosterone. However, the team needed to know whether this increase was or was not caused by the reduction in feeding.
With a bit of trickery up their sleeves, the researchers decided to swap out chicks of different ages between nests. They did so to see how this affected both parental care and the time chicks took to fledge.
"Manx Shearwaters don't recognize their own offspring, but will simply go back to the same nest after they've gathered food. They have one chick, which makes the interactions between parent and offspring easier to study," explains lead researcher, Dr Keith Hamer. "We swapped chicks which were between 10 days and two weeks apart in age, to see what impact it would have. We wanted to find out whether parents and chicks were responding to each other's behavior, or whether each was acting independently."
The team discovered that adults lowered their food provisions after roughly 60 days of raising a chick, regardless of the chick's stage of development.
Although females are more apt than males to adjust their feeding levels to how much their chicks beg for food, after approximately 60 days both parents start to ignore their pleas. This held true whether parents were feeding their own chicks, or foster-chicks of different ages.
The surge in corticosterone took place over the last few weeks before chicks fledged at about 70 days of age. This held true even when chicks had been fostered by parents at a different stage of the feeding cycle, so this was clearly independent of the parent's behavior and any reduction in food.
"Our findings show that young Manx Shearwaters leave home of their own accord when their corticosterone levels have reached a peak rather than as a result of changes in parental behavior," says Dr Hamer. "Both parents and chicks need large energy reserves for their arduous migration across the Atlantic to South and Central America, and parents seem to reduce how much they feed their young simply to protect themselves."
"Unlike some other bird species, which let their offspring dictate the level of care, seabirds appear to weigh up the cost of a chick fledging underweight against the greater cost of losing the chance to breed again," he adds. "Manx Shearwaters have a breeding life of around forty years, so parents pay a high cost if they end the season too weak to complete their own migration."
The study was published online in Behavioral Ecology on July 5, 2012.