Foraging of a coastal seabird: flight patterns and movements of breeding Cape gannets Morus capensis

Abstract

Cape gannets Morus capensis are predatory seabirds in the coastal waters of southern Africa where they feed on commercially important fish species. Using a combination of intensive monitoring at nest sites, tracking of radio-tagged birds and diet sampling, we determined the foraging ranges and foraging areas used by breeding gannets, and whether links existed between the broad-scale spatial distribution of foraging birds and the distribution of prey or the predominant wind regime. A total of 270 Cape gannets dispersing to forage from Malgas Island, South Africa, were tagged over three consecutive breeding seasons. Modal durations of foraging flights were six or 24 hours, depending on whether birds returned on the same day they left or remained at sea overnight. Few birds remained continuously at sea over two consecutive nights. Non-radiotagged birds more frequently undertook shorter foraging trips than radio-tagged birds, indicating a behavioural response to handling. Some 23% of radio-tagged gannets triangulated throughout a complete foraging trip foraged within a maximum of about 60km of Malgas Island, 44% foraged between about 60km and 120km of the island and the remaining 33% flew beyond 120km, travelling a total of at least 240km. Flight directions of gannets departing from the island were non-random in two of the three seasons. Return flight directions were non-random in all three seasons. Most foraging flights were to the south-west of the island, birds generally returning with the prevailing wind and from the same general direction in which they departed. Birds returning with saury Scomberesox saurus did so significantly more frequently from a west-south-west direction. Birds feeding on the other two prey species were equally likely to return from any direction. Under average conditions, the energy benefit associated with returning under load with a tailwind as opposed to a headwind was equivalent to 12% of the average stomach sample mass. The observed distribution of flights probably reflects the large area of suitable foraging habitat to the south and south-west of the island and the energetic advantages of returning with the prevailing wind.

African Journal of Marine Science 2005, 27(1): 239–248