Parental activity budgets and energy expenditures of Northern Gannets (Sula bassanus)

Activity budgets and energy expenditures were measured simultaneously using doubly labeled water and activity timers on 19 free-living Northern Gannets (Sula bassanus) rearing chicks at Funk Island, Newfoundland, in 1984-1985. Gannets expended a mean of 5140±777 kJ·d⁻¹ (n＝30 sample intervals), or 6.9 x basal metabolic rate (BMR), about twice the level predicted from allometric equations. Daily energy, expenditures (DEE) increased with the proportion of time spent away from the nest ('foraging'). Multiple regression analysis indicated a nest-site metabolic rate (EMR) of 112±36.5 kJ·h⁻¹, a flying metabolic rate of 382±115 kJ·h⁻¹ and a surface swimming metabolic rate (including resting on water) of 159±96.5 kJ·h⁻¹. Time budgets were approximately evenly divided among nest attendance, swimming and flying; 19±8% of DEE was expended in nest attendance, 25±10% swimming and 56±12% flying (n＝ll). Study birds spent 19% less time in nest attendance than control animals, and time spent resting on water was probably inflated due to handling. Gannets obtained a mean of 2.5±0.7 kJ of food per kJ expended foraging. Thermoregulatory requirements and high costs for plunge-diving could help to explain the high energy expenditures. It is hypothesized that the 4 x BMR maximum sustainable level of energy expenditure for free-living animals may actually be closer to 4 x standard metabolic rate. DEE did not differ between sexes and decreased with chick age, probably due to decreased foraging demands on the parents. Chick-feeding in gannets may be constrained by energy expenditures and/or digestion as opposed to time budgets or food shortages