At-sea-activity and foraging efficiency in chick-rearing northern gannets Sula bassana:: a case study in Shetland

Chick-rearing northern gannets Sula bassana from Hermaness, Shetland (UK), were equipped with both stomach temperature loggers and external temperature loggers (attached to the leg). Detrimental device effects on the birds could not be detected. Three complete data sets covering several foraging trips by 3 birds showed that the logger-equipped birds spent 39 to 49% of their time in the colony, 22 to 30% flying and 22 to 34% swimming. Foraging trips lasted between 2 h 45 min and 27 h 08 min, with a mean length of 13 h. Maximum foraging range was estimated to be 128 km. Dives lasted between 1 s and 7.5 s, with a mean of 4.4 s. Median food quantity swallowed per feeding event was 101 g (n = 32), with 745 g being the maximum. No foraging activity occurred at night. Catch per unit effort was assessed to range between 0.9 and 2.8 g fish min(-1) flying and between 0.5 and 1.3 g fish min(-1) at sea. Foraging efficiency varied between 0.6 (negative energy budget) and 1.5 (positive energy budget). More data on foraging efficiency, preferably from different colonies and different years, could show how efficiently this top predator utilises food resources.