Abstract: 1. For seabirds that forage at sea but breed fasting on land, successful reproduction depends
on the effective management of energy stores. Additionally, breeding often means
aggregating in dense colonies where social stress may affect energy budgets.
2. King penguin (Aptenodytes patagonicus) males fast for remarkably long periods (up to 1.5
months) while courting and incubating ashore. Although their fasting capacities have been
well investigated in captivity, we still known very little on the energetics of freely
breeding birds.
3. We monitored the heart rate (HR; a proxy to energy expenditure), body temperature, and
physical activity of male king penguins during their courtship and first incubation shift in
a colony of some 24,000 freely breeding pairs. Males were either breeding under low but
increasing colony density (early breeders), or at high and stable density (late breeders).
4. In early breeders, daily mean and resting HR decreased during courtship but increased
again 3 days before egg-laying and during incubation. In late breeders, HR remained
stable throughout this same breeding period. Interestingly, the daily increase in resting HR
we observed in early breeders was strongly associated with a marked increase in colony
density over time. This finding remained significant even after controlling for climate
effects.
5. In both early and late breeders, courtship and incubation were associated with a
progressive decrease in physical activity, whereas core body temperature remained
unchanged.
6. We discuss the roles of decreased physical activity and thermoregulatory strategies in
sustaining the long courtship-incubation fast of male king penguins. We also draw
attention to a potential role for conspecific density in affecting the energetics of breeding
fasting seabirds, i.e. a potential energy cost to coloniality.