Testing olfactory foraging strategies in an Antarctic seabird assemblage.

Procellariiform seabirds (petrels, albatrosses and shearwaters) forage over thousands of square kilometres for patchily distributed prey resources. While these birds are known for their large olfactory bulbs and excellent sense of smell, how they use odour cues to locate prey patches in the vast ocean is not well understood. Here, we investigate species-specific responses to 3-methyl pyrazine in a sub-Antarctic species assemblage near South Georgia Island (54 degrees 00 ' S, 36 degrees 00 ' W). Pyrazines are scented compounds found in macerated Antarctic krill (Euphausia superba), a primary prey item for many seabird species in this region. To examine behavioural attraction to this odour, we presented birds with either scented or 'unscented' vegetable oil slicks at sea. As a positive control for our experiments, we also compared birds' responses to a general olfactory attractant, herring oil. Responses to pyrazine were both highly species specific and consistent with results from earlier studies investigating responses to crude krill extracts. For example, Cape petrels (Daption capense), giant petrels (Macronectes sp.) and white-chinned petrels (Procellaria aequinoctialis) were sighted at least 1.8-4 times as often at pyrazine-scented slicks than at control slicks. Black-browed albatrosses (Diomedea melanophris) were only sighted at pyrazine-scented slicks and never at control slicks. Wilson's storm-petrels (Oceanites oceanicus), black-bellied storm-petrels (Fregetta tropica), great shearwaters (Puffinus gravis) and prions (Pachyptila sp.) were sighted with equal frequency at control and pyrazine-scented slicks. As expected, responses to herring oil were more common. With the exception of great shearwaters (Puffinus gravis), each of these species was sighted up to five times as often at slicks scented with herring oil compared with control slicks. Together, the results support the hypothesis that Antarctic procellariiforms use species-specific foraging strategies that are inter-dependent and more complex than simply tracking prey by scent.