Searching the vast expanses of the Antarctic Southern Ocean for patches of krill is a challenging task, but tubenosed seabirds like petrels and albatrosses are outstanding at locating the tasty crustaceans they depend on for food. Gabrielle Nevitt from the University of California and her collaborator Keith Reid from the British Antarctic Survey also faced challenging circumstances when they decided to study these birds' hunting strategies, confronting high seas and miserable weather with binoculars in hand to investigate how marine birds use scent cues to locate their prey. Nevitt explains, `I have always been fascinated by tubenosed, or procellariiform, seabirds–they are my favourite animals on the planet!'adding that `Tubenoses are thought to hunt either by smell or by a combination of olfactory and visual cues, possibly detecting their prey from as far away as ten kilometres'.

Nevitt explains that a variety of odours signal the presence of food in the ocean, creating an olfactory landscape. For example, dimethyl sulphide (DMS)is released when crustaceans graze on plant-based plankton, whereas pyrazine is given off by krill crushed during seabird feeding. Knowing that smaller species of seabird like storm-petrels and prions prefer the smell of DMS,Nevitt says `I wondered whether some species of bird are particularly attracted to [plankton's] DMS, while others respond to the [pyrazine] odour of krill' (p. 3537).

So Nevitt and Reid set out on the British Antarctic Survey ship RRS James Clark Ross to test which species were attracted to pyrazine, by floating scented vegetable oil slicks next to the boat to see which birds were attracted to them. Settling down with their binoculars for some serious bird watching, the team found that only certain species were lured to the boat by the scent of pyrazine, including Cape petrels, Giant petrels and Black-browed albatrosses. Intriguingly, these birds are also noted for their aggression and tend to feed in large, mixed-species groups. Nevitt explains that aggressive tubenoses were previously thought to hunt mainly by using visual clues, such as spotting other birds at krill feeding frenzies. But the odour of pyrazine also seems to attract the birds, and can be detected over much greater distances.

In the light of her results, Nevitt suspects that these aggressive species deliberately sniff out the smell of crushed krill, usually found where other birds are feeding. They then muscle in on the food, fighting off any competition. Smaller, more passive species prefer to hunt opportunistically,using the scent of DMS released as krill feast to lead them to their dinner. Nevitt's results suggest that not only are scent and visual cues important when tracking prey in the open ocean, but social cues–such as the presence of other birds and behavioural patterns–also play a role.

After spending up to ten hours a day watching birds, Nevitt's binoculars beckon again. For her next Antarctic adventure, she wants to study the development and evolution of procellariiform birds. `I want to figure out why some procellariiform species appear to be really good smellers, whereas others, such as albatrosses, rely more heavily on visual cues', she says.

References

Nevitt, G., Reid, K. and Trathan, P. (
2004
). Testing olfactory foraging strategies in an Antarctic seabird assemblage.
J. Exp. Biol.
207
,
3537
-3544.