Does the foraging strategy of adult Short‐tailed Shearwaters cause obesity in their chicks?

Abstract

The chick provisioning behaviour of Short‐tailed Shearwaters Puffinus tenuirostris breeding at the northern edge of their distribution on Montague Island, New South Wales, was examined in February and March 1997. The duration of individual foraging trips of parents, weight changes of adults and chicks, and meal sizes delivered to chicks were determined. It was found that individual parents mixed a long foraging trip to Antarctic waters (14.4±2.0 days) with one to three short foraging trips (1.36±0.7 days, mode=1 day). Adults gained body mass on long trips and lost weight on short trips. The size of meals fed to the chicks was significantly greater after a long trip (161±21 g) than after a short trip (135±28 g), although short trips increased the overall chick feeding frequency. The variable number of short trips made by adult Short‐tailed Shearwaters and the relationship between short trips and adult body condition were consistent with current life‐history theory: adults do not sacrifice their own body condition to increase food delivered to their chicks. Modelling revealed that this dual foraging strategy inevitable leads to chicks enduring long intervals between meals. These long intervals may have led to the evolution of an over‐feeding strategy by parents and the nestling obesity reported in this shearwater. The durations of the long trips from Montague Island were significantly greater than those for Short‐tailed Shearwaters breeding at the centre of their distribution in Tasmania, although there was no significant difference in the length of short trips. A commitment to feed regularly in Antarctic waters may explain why the breeding distribution of this species does not extend much further north.