The surge in corticosterone took place over the final 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 was clearly independent of the parent's behaviour 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 behaviour," 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."
|Contact: Dr. Keith Hamer|
University of Leeds