Energetic costs of foraging in breeding Cape gannets Morus capensis

Seabirds fly considerable distances during the breeding season in search for food for themselves and their young. Variation in the distance from the breeding colony to the offshore food resources is expected to impact the energy spent on foraging trips. In 2005-06 and 2006-07 we studied foraging behaviour, derived time budgets during foraging trips (commuting, hunting or drifting on the sea surface) and measured the associated energy expenditure in 2 colonies of breeding Cape gannets Morus capensis. Around Ichaboe Island (Namibia) the winds were stronger and more variable than at Malgas Island (South Africa). Gannet foraging trip duration did not vary between the islands, but at Ichaboe gannets spent more time on hunting and less time drifting on the sea surface compared to Malgas birds. Gannets from Malgas made more dives during foraging trips than Ichaboe gannets (75 and 43 dives respectively). Energy expenditure during foraging trips (TEE) was estimated on average at 4203 kJ d–1 (±693, n = 27), which was 5.5 × basal metabolic rate (BMR), and did not differ between the islands. Energetic costs of foraging increased with wind speed and the fraction flying during foraging trips. The average flight costs were estimated at 85 W, after correction for wind speed. The increased energetic cost during foraging at Malgas was associated with the large number of dives and less profitable winds: taking off after each plunge-dive would be more costly in weaker winds. The fact that TEE did not differ between the islands might suggest that Cape gannets at both islands were foraging at the boundaries of their sustainable energetic expenditure.