PREY RESOURCES, COMPETITION, AND GEOGRAPHIC STRUCTURE OF KITTIWAKE COLONIES IN PRINCE WILLIAM SOUND

Abstract

Mostly indirect evidence has shown that the size of bird colonies is often related positively to the size of foraging areas, in turn an indirect measure of prey availability. We report here results of an unusual opportunity to directly investigate the relationships among the size and distribution of colonies of Black-legged Kittiwakes (Rissa tridactyla), the degree of overlap of colony-specific foraging areas, and the availability of kittiwake prey, principally Pacific herring (Clupea pallasi) and sand lance (Ammodytes hexapterus), in Prince William Sound, Alaska, 1995–1999. Aerial surveys to assess prevalence of prey schools visible at the surface (the kittiwake mode of prey search) allowed quantification of prey availability in foraging areas. Foraging areas and foraging ranges were determined by radio telemetry. Large colonies occurred as far apart as physically possible and not within foraging range (∼40 km) of one another; only small colonies occurred within the respective ranges of large colonies. Foraging range, foraging trip duration, and area of foraging grounds were related positively to colony size. Down to the finest scale (1.5 nautical miles, or 2.8 km), the presence–absence of foraging kittiwakes was related to where prey schools were likely to occur, rather than there being a direct relationship between densities of kittiwakes and observed prey schools. Annual variation in school density did not alter the relationship between colony size and size of foraging grounds, i.e., more abundant prey did not reduce the size of the foraging ground. Importantly, the quality of foraging grounds (prevalence of fish schools at the surface) decreased with colony size. We conclude that the density of foraging kittiwakes within Prince William Sound has reached a limit: additional birds would compromise access to prey, probably by passive foraging interference. This pattern would explain why overall numbers of breeding kittiwakes in the Sound have remained relatively stable during the past few decades, whereas several colonies have dramatically increased in size and others have decreased or disappeared. Corresponding Editor: C. R. Brown