Corticosterone at fledging depends on nestling condition, not on parental desertion

Abstract

In some burrow-nesting birds, fledging is preceded by a strong rise in circulating corticosterone levels. Because the parents may desert nestlings in these species, this rise may be internally triggered or it may be a response to failing adult attention. We followed nestling thin-billed prions throughout the pre-fledging period in three seasons, 2005–2007. These seasons had contrasting oceanographic conditions, the sea surface temperature being warm in 2005, intermediate in the fledging period in 2006, and cold in 2007. Accordingly, the food availability, reflected in chick peak masses and begging rates, was low throughout 2005, intermediate during 2006, and high throughout 2007. We observed that most caring parents attended thin-billed prion nestlings until they were fledgings in the seasons of food scarcity during 2005 and 2006. In contrast, chicks were left alone for a mean of 4 days before fledging in 2007. The pre-fledging rise in corticosterone was lowest in the good year of 2007, even though most chicks were not attended during the pre-fledging period in that year. Our results thus indicate that CORT at fledging depends on nestling age and condition, not on parental desertion.