Feeding Associations between White-Fronted Terns and Hector's Dolphins in New Zealand

Abstract

I investigated the relationship between White-fronted Terns (Sterna striata) associating with Hector's dolphins (Cephalorhynchus hectori) while feeding in the near-shore environment of New Zealand. Associations were observed only in the austral spring and early summer. At this time up to 15.7% of all dolphin groups were accompanied by terns. The birds usually joined surface-feeding dolphins with single terns apparently hovering over single dolphins. Associated tern numbers were positively correlated with dolphin group size and increased significantly with the duration of the association. Terns were more often associated with significantly larger dolphin groups and under calmer sea conditions. Water clarity, however, appeared to have little influence in the birds' decision whether or not to associate with a group of foraging dolphins. The occurrence of feeding associations appeared to be restricted by the occurrence of small, inshore moving fish species. The associations are likely to constitute facultative commensalism by the birds and may be particularly advantageous during breeding when energetic demands are high.