The Ecology of Seabird Feeding Flocks in Alaska

--Seabirds commonly gather into mixed-species flocks to feed on fish schools and other concentrations of prey. We group Alaskan and Washington seabird feeding flocks into three types on the bases of flock size and longevity and the nature of the food source. Small, short-lived flocks over tightly clumped prey are called Type I; larger (5,000+ individuals), longer-lasting flocks over less tightly clumped and less reactive prey are called Type II; Type III flocks form where zooplankton and other organisms are concentrated by downwelling. Birds participating in the flocks are assigned to four functional groups (some species fit into two groups): catalysts (larids and shearwaters) are highly visible birds that other birds watch and follow to food sources; divers (alcids, loons, cormorants) exploit the food sources underwater by pursuit diving; kleptoparasites (jaegers and gulls) steal food from other flock members; and suppressors (shearwaters and cormorants) interfere behaviorally with the feeding of other flock members by reducing the effective prey availability. Most flocks occurred within a few kilometers of shore. Type I flocks on the Washington coast averaged larger, lasted longer, and contained more species than Alaskan Type I flocks. The Washington and Alaska flocks contained about the same number of locally breeding species, but the Washington flocks also contained several migrant species that breed elsewhere in North America. Both contained shearwaters, migrants from the southern hemisphere, but the shearwaters were much more important in the Alaskan flocks. Black-legged Kittiwakes and shearwaters (catalysts) initiated most Alaskan flocks and were important in the development of flocks initiated by other birds. Once a flock was initiated, it grew until the food source became unavailable or until the local pool of prospective flock members was exhausted. The divers were able to discriminate from considerable distances between kittiwakes feeding on single fish and kittiwakes feeding on fish schools and approached only the latter. The various species tended to occupy characteristic positions within Type I flocks. Gulls and kittiwakes were central, and the various divers took peripheral positions. Kleptoparasitism by jaegers did not appear to influence Type I flock organization. Shearwaters, the most important suppressors, sometimes pursuit-plunged into fish schools and euphausiid shoals in such numbers that the prey concentrations were drastically reduced, scattered, or driven downward in seconds, and other birds were then unable to feed. Type II flocks were divisible into two groups, one consisting largely of kittiwakes and shearwaters and feeding on capelin, and the other dominated by shearwaters and feeding on pelagic crustaceans. Kleptoparasitism by Pomafine Jaegers in the capelin-based Type II flocks was frequent and differed from the kleptoparasitism of solitary birds in that the jaegers preferentially attacked birds carrying fish in their bills. Suppression appeared unimportant in capelin-based Type II flocks but probably kept alcids and gulls from joining the crustacean-based flocks. In some island groups Type III flocks occurred daily. They were less regular in structure and composition than Type I or Type II flocks. Kleptoparasitism by gulls and kittiwakes tended to keep puffins and other alcids on the edges of the flocks. The alcids' underwater approaches to the fish schools from the sides may have tended to keep the schools compact and near the surface. It has been hypothesized that the 437 The Auk 98: 437-456. July 1981 438 HOFFMAN, HEINEMANN, AND WIENS [Auk, Vol. 98 antipredator function of schooling by baitfish involves predator satiation and the difficulty of locating schools. Schooling does not function as. a deterrent to aerial predators in the same way that it does to swimming ones, however. Either birds are less important as predators, or schooling confers a different advantage in escaping aerial predation. Apparently, fish schools can escape rather quickly from bird flocks by descending away from the surface out of visual contact. Received 8 January 1980, accepted 12 February 1981. FISH-EATING and other seabirds in most of the world's oceans exploit fish schools and other clumped food sources in multispecies flocks. These flocks often include several species that feed differently (Gould 1971, Scott 1973, Sealy 1973) but sometimes in a complementary manner. If indeed the birds use complementary tactics when feeding together, the assemblages may be tightly interacting, coevolved systems. We studied the arian communities of the temperate and subarctic northeast Pacific Ocean to discern how intense, consistent, and obligatory such feeding interactions are. We surveyed the geographic and hydrographic distributions of flocks in our study areas and characterized them by structure, species composition, and food