STOMACH OIL AND THE ENERGY BUDGET OF WILSONS STORM-PETREL NESTLINGS

Aspects of chick-provisioning in Wilson's Storm-Petrel were measured a t Palmer Station, Antarctica. Chicks received meals averaging 9.2 g (2 1% of adult body mass) with a mean frequency of 1.1 feeds/chick.day. F eeding frequency decreased as chicks grew, but meal mass remained rela tively constant throughout the fledging period. Free lipids accounted for an average of 24% of the mass and over 60% of the energy in a meal . The energy density of the meals averaged 16.1 kJ/g, nearly triple th at of whole prey. The estimated rates at which gross and metabolizable energy were delivered to the chick averaged 163 and 147 kJ/days, resp ectively, over the 60 day fledging period. Low nest temperatures resul ted in high energy requirements by the chicks. Rates of respiratory en ergy metabolism averaged 133 kJ/day over the hedging period and reache d peak levels of similar to 160 kJ/day. Rates of energy deposition in tissues averaged only 18.4 kJ/day day with a peak rate of similar to 5 7 kJ/day. Thus, total metabolizable energy requirements of the chick a veraged 151 kJ/day, which is within 3% of estimated metabolizable ener gy intake. Wilson's Storm-Petrel chicks have energy requirements nearl y double those of Leach's Storm-Petrel chicks which grow in a more tem perate climate. Wilson's Storm-Petrel adults appear to meet demands of their chicks with a high frequency of meal delivery and high energy d ensity of meals compared to those of Leach's Storm-Petrel. A compariso n of the rates of energy utilization by foraging adults of the two spe cies suggests that the high energy demands of the Wilson's Storm-Petre l chicks have selected for a foraging strategy in adult Wilson's Storm -Petrels that is expensive in terms of energy and time. Low air temper atures, a short summer season, and the presence of diurnal predators o n the breeding grounds are among the factors which have shaped the chi ck-provisioning strategy of this species. The data suggest that Wilson 's Storm-Petrels could not breed successfully in the Antarctic without the ability to produce stomach oils.