Foraging effort and prey choice in Cape gannets

Abstract

In order to test the hypothesis that sardine Sardinops sagax are the preferred prey of Cape gannets Morus capensis, the link between foraging effort and prey choice was evaluated by simultaneously monitoring the activity and the diet of adult birds attending chicks at Bird Island, Algoa Bay, South Africa. Foraging trip durations were bimodally distributed. Most foraging trips of Cape gannets were completed within 24 h. Metered gannets spent c. 40% of this time flying. In all, nine prey species were recovered from the stomachs of metered birds. The diet was dominated by the commercially important sardine Sardinops sagax and anchovy Engraulis capensis. Food mass intake by foraging Cape gannets was not correlated with foraging trip duration or time flying, consistent with a patchy distribution of food. Cape gannets returning with sardine tended to have shorter foraging trips and spent significantly less time flying than birds returning with other prey, thereby maximizing net energy intake. Sardine seem to be the preferred (most profitable) prey and, consequently, their relative proportion in gannet stomachs may reflect their absolute availability at spatial scales equivalent to the bird's foraging range.