Stomach Oil and the Energy Budget of Wilson's Storm-Petrel Nestlings

Aspects of chick-provisioning in Wilson's Storm-Petrel were measured at Palmer Station, Antarctica. Chicks received meals averaging 9.2 g (21% of adult body mass) with a mean frequency of 1.1 feeds/chick day. Feeding frequency decreased as chicks grew, but meal mass remained relatively 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 that of whole prey. The estimated rates at which gross and metabolizable energy were delivered to the chick averaged 163 and 147 kJ/days, respectively, over the 60 day fledging period. Low nest temperatures resulted in high energy requirements by the chicks. Rates of respiratory energy metabolism averaged 133 kJ/day over the fledging period and reached peak levels of ∼160 kJ/day. Rates of energy deposition in tissues averaged only 18.4 kJ/day with a peak rate of ∼57 kJ/day. Thus, total metabolizable energy requirements of the chick averaged 151 kJ/day, which is within 3% of estimated metabolizable energy intake. Wilson's Storm-Petrel chicks have energy requirements nearly double those of Leach's Storm-Petrel chicks which grow in a more temperate climate. Wilson's Storm-Petrel adults appear to meet demands of their chicks with a high frequency of meal delivery and high energy density of meals compared to those of Leach's Storm-Petrel. A comparison of the rates of energy utilization by foraging adults of the two species suggests that the high energy demands of the Wilson's Storm-Petrel chicks have selected for a foraging strategy in adult Wilson's Storm-Petrels that is expensive in terms of energy and time. Low air temperatures, a short summer season, and the presence of diurnal predators on the breeding grounds are among the factors which have shaped the chick-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.