Flocking Outside Nesting Colonies in Glaucous-Winged Gulls.

Abstract

Flocking outside nesting colonies in Glaucous-winged Gulls (Larus glaucescens) was investigated to examine whether flocks were functional. Localized sites providing safety from predators, shelter from wind, perches conducive to detection of fish schools, and abundance of prey were considered possible causes for nonfunctional aggregations. Minimization of cost-benefit ratios through cooperative procurement of prey was considered one possible function of flocks. Lowering odds of falling prey to predators through cooperative defense, using conspecifics in flocks as shields, shared vigilance, and association with other attractive prey was considered another possible function. Seven hypotheses were generated to test possible causes for aggregations and functions of flocking. Data were collected from July 1980 to July 1981 on Lopez Isl and , Washington. Not all flock sites were localized and flocks failed to perch consistently in locations judged to be safest available sites. During strong winds, flocks perched more often in unsheltered than sheltered sites. Most individuals of flocks perched in sites conducive to detection of fish schools failed to leave flock sites to prey on fishes. Therefore, safety from predators, shelter from wind, and perches for detection of fish schools were not acceptable causes for aggregations. Gulls foraged in locations with abundance of prey but maintained r and om dispersons and principally large distances from one another. In addition, foraging gulls as well as gulls that preyed on fish schools exhibited no cooperative but agonistic, if any, interactions, precluding cooperative procurement of food. Therefore, foraging gulls and gulls preying on fishes were judged to be nonfunctional aggregations. Glaucous-winged Gulls did not practice cooperative defense outside nesting colonies. Competition among gulls for central locations in flocks, suggesting the use of conspecifics at peripheries as shields, was negligible. Gulls formed airborne flocks in response to organisms that elicited alarm cells, and during formations of perched flocks, most gulls alighted within 2 m of conspecifics. Inevitably, the odds of falling prey to predators were lower for flocked than solitary gulls. Therefore, perched and airborne flocks were judged to be functional and association with other attractive prey to reduce susceptibility to predation was judged to be the most plausible function of such flocks.