OBSERVATIONS OF MULTISPECIES SEABIRD FLOCKS AROUND SOUTH GEORGIA

Abstract

OBSERVATIONS OF MUL TISPECIES SEABIRD FLOCKS AROUND SOUTH GEORGIA NANCY M. HARRISON,1.3 MICHAEL J. WHITEHOUSE/ DENNIS HEINEMANN, 1•4 PETER A. PRINCE,2 GEORGE L. HUNT JR., 1 AND RICHARD R. VEIT 1 .s 'University of California, Irvine, California 92717 USA, and British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, U.K. ABSTRACT.-We observed 195 multispecies feeding flocks of birds near Bird Island, South Georgia, between 4 February and 5 March in 1986. Up to 19 seabird species plus Antarctic fur seals (Arctocephalus gaz.ella) were involved. Black-browed Albatross (Diomedea melanophris), Macaroni Penguins (Eudyptes chrysoloplius), and prions (Pachyptila spp.) were the most abun- dant species. Most flocks were in the vicinity of a large inshore patch of Antarctic krill (Euphausia superba), and most observed feeding was by Black-browed Albatross, which ap- peared to initiate and dominate the flocks. Short feeding bouts by surface-feeding species appeared to be coupled with the surfacing of penguins and seals. We conclude that foraging penguins and seals caused prey to approach the surface where near-surface foraging species could feed. Received 2 July 1990, accepted 2 March 1991. BIRDS, mammals, and fish commonly form multispecies feeding assemblages at sea (Mur- phy 1936, Bailey 1966, Ashmole and Ashmole 1967, Duffy 1983). Some assemblages result from the independent attraction of several species to a prey concentration with little interaction among the various predators (Ryder 1957, Evans 1982). However, most authors argue that many species benefit from feeding in groups (Sealy 1973, Brown 1980, Enticott 1986, Pierotti 1988b ), because others locate or concentrate prey. As- semblages may comprise species that play par- ticular roles in the initiation, maintenance, and disruption of the group (Hoffman et al. 1981, Porter and Sealy 1982). Species that use different foraging methods (e.g. aerial vs. diving predators) differ in their prey-finding capabilities. Black-legged Kitti- wakes (Rissa tridactyla) are catalysts to flock for- mation in Alaskan waters (Hoffmann et al.1981), where they find concentrations of prey and alert other birds to its location by their conspicuous feeding behavior. Diving predators are able to find subsurface prey and may influence the dis- ' Present address: Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, U.K. •Present address: Manomet Box, P.O. Box 936, Ma- nomet, Massachustts 02345 USA. s Present address: Department of Zoology, NJ-15, University of Washington, Seattle, Washington 98195 USA. tribution of prey at the surface (Brown 1980, Grover and Olla 1983). Marine mammals and penguins have been observed driving fish to the surface while feeding (Jehl 1974, Boswall and Maciver 1975). Surface-feeding seabirds may be attracted by the presence of other predators as much as by the prey itself. Feeding assemblages appear to occur when resources are clumped. The degree to which predators are interactive may represent an adaptive response to a high degree of resource patchiness (Hoffman et al. 1981, Duffy 1983). Antarctic krill (Euphausia superba) is one of the most highly aggregated prey of birds and mam- mals in Antarctica (Hamner et al. 1983), and krill is an important component in the diet of most predators and an essential resource for many seabird species (Croxall and Prince 1987). Although vastly abundant, the tendency of krill to form dense subsurface swarms may make it difficult for many seabird species to locate and track over time. Feeding in multispecies flocks may be a beneficial behavior for seabirds, par- ticularly surface-feeding species, in Antarctica. Although krill is a major component in the diets of seabirds breeding on South Georgia (Croxall and Prince 1980, 1987), little is known of the means by which seabirds capture !~rill. Observations of Antarctic birds feeding at sea are few and are !biased towards the scavenging behavior of some petrels and albatross near ships (Griffiths 1982, Weimerskirch et al. 1986, Harp- The Auk 108: 801-810. October 1991