Behavioural responses by foraging petrels to swarms of Antarctic Krill Euphausia superba

Abstract

Petrels feeding on Antarctic Krill Euphausia superba use two basic strategies to locate prey patches. One strategy is area-restricted search, in which individuals interrupt straight-line motion on detecting prey, and then circle about. This increased rate of turning near prey increases the likelihood of detecting additional patches, since patches tend to be clumped in space. A second strategy is local enhancement, in which individuals monitor the behaviour of others, and converge on those individuals that display behaviour indicative of feeding. In this paper, attention is focused on the first strategy, area restricted search, and evidence of increased turning rates in the vicinity of krill patches is sought. The hypotheses tested were first, that six species of krill-feeding petrels engage in area restricted search, and then that the tendency for seabirds to either sit on the water, patter with their feet, or seize prey was correlated with the distance to the nearest krill patch. Seabird abundance and behaviour was paired with krill abundance east of South Georgia (55°S, 35°W) in June 1993. Three species, Cape Petrel Daption capense, Northern Giant Petrel Macronectes halli and Southern Giant Petrel M. giganteus, changed their direction of flight significantly more often when near krill swarms than when in krill-free areas, indicating that they used area restricted search. Cape Petrels were also significantly more likely to sit on the water and to patter along the surface when close to krill swarms.