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OBSERVATIONS OF A MIXED-SPECIES FLOCK OF BIRDS FORAGING ON EUPHAUSIIDS NEAR ST. MATTHEW ISLAND, BERING SEA GEORGE L. HUNT ]R.,1 NANCY M. HARRISON, 1 WILLIAM M. HAMNER,2 AND BRYAN 5. 0BST 2 'Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA, and 'Department of Biology, University of California, Los Angeles, California 90024 USA ABSTRACT.-We observed mixed-species flocks of birds foraging for euphausiids near Cape Upright, St. Matthew Island. Murres (Uria spp.) predominated, and the number present was equivalent to 79% of the murres breeding at Cape Upright, or 11% of the total St. Matthew population. All bird species actively feeding were catching the euphausiid Thysanoessa raschii. SCUBA divers determined that murres were diving deeper than 30 m; schools of euphausiids were not observed above that depth. Injured and disoriented euphausiids were present near the surface where surface-feeding Black-legged Kittiwakes (Rissa tridactyla) captured them. Received 20 April 1987, accepted 16 January 1988. LARGE mixed-species seabird flocks feed in the vicinity of the Pribilof Islands (Hunt et al. 1978, Hunt unpubl. data) and at St. Matthew Island. The flocks are com posed typically of large numbers of diving species, the majority murres (mostly Thick-billed Murres, U. lomvia, with some Common Murres, U. aalge), with smaller numbers of Crested Auklets (Aethia cristatella), Least Auklets (A. pusilla), Parakeet Auklets (Cy- clorrhynchus psittacula), and Horned Puffins (Fra- tercula corniculata). In addition to these diving species, several surface-foraging species are regularly in attendance. These include North- ern Fulmars (Fulmarus glacialis), red Phalaropes (Pha/aropus fulicaria), and Black-legged Kitti- wakes (Rissa tridactyla). For these mixed-species flocks, when prey identity was known, eu- phausiids were the primary food item in the guts of all species. Euphausiids occur in the diets of most species of seabirds in the Bering Sea (Sanger and Baird 1977, Hunt et al. 198la). These crustaceans are particularly important for migratory Short-tailed Shearwaters (Puffinus tenuirostris) and for several species of breeding alcids (Bedard 1969, Hunt et al. 198l a, Schneider et al. 1986). During the day euphausiids usually migrate away from the surface, and in the shallow (30-60 m) shelf re- gions of the Bering Sea probably aggregate close to the bottom (S. Smith pers. comm.), where they are relatively inactive and densely packed (Mauchline and Fisher 1969). The diurnal cap- ture of euphasuiids by birds able to dive to depths where euphausiids concentrate is not surprising. However, the regular occurrence of euphausiids in the gut contents of diurnal, sur- face-foraging Black-legged Kittiwakes raises the question of how they obtain this prey. On 21, 27, and 28 June 1984 we encountered large mixed-species flocks near St. Matthew Is- land over water approximately 62-64 m deep (Fig. 1). We investigated one segment of the flock on 27 June. We mapped the distribution of foraging birds, collected biirds for stomach- content analysis, and put SCUBA divers in the water to observe underwater prey distribution and bird foraging. METHODS We counted birds along a series of transects east of Cape Upright (Fig. 1). Continuous counts were made of all birds within a 90 arc from the bow to the beam and extending to 300 m from the ship on the side with the best visibility. The sea was calm, and visi- bility was excellent for counting birds. Birds on the water were tallied separately from flying birds, and only birds on the water were used to determine for- aging distributions. Data were entered directly into a microcomputer custom programmed with data-en- try software (l:Jpdegraff and Hunt 1985) that permit- ted continuous records of bird numbers. From approximately 1500 to 1900 local time, we collected actively foraging seabirds. Diving species observed returning to the surface were selected, as were surface-foraging species seen dipping or peck- ing at the surface. We removed stomachs from birds within 1 h of collection and preserved the crops, stomachs, and their contents in 80% ethyl alcohol. We obtained information on the near-surface and The Auk 105: 345-349. April 1988

Observations of a Mixed-Species Flock of Birds Foraging on Euphausiids near St. Matthew Island, Bering Sea