Thermal Stress and Predation: Influences on the Structure of a Gull Colony and Possibly on Breeding Distributions

The arrangement of nesting territories of Larus occidentalis livens on beaches is commonly linear, and most territories are adjacent to water. This arrangement differs from clustered colonies typical of most gulls. Two hypotheses are advanced to explain this unusual nest arrangement: it may provide access to the intertidal zone for foraging or access to water for thermoregulation. Foraging activity in the intertidal zone is of only modest or secondary importance. In contrast, thermal stress is great and access to water for thermoregulation appears to be essential. Due to the organization of the nesting colony, adults can leave their nests and go to the water during long incubation and brooding bouts and still return quickly to prevent nest predation by ravens or other gulls. The linear arrangement of nests is less pronounced on broad beaches or peninsulas, although close proximity to the water persists. The absence of gulls as breeding species on most tropical islands, which may be related to heat stress and associated problems of predation or overexposure of eggs to the sun, rather than to shortage of food, is discussed. Adaptations by which some species may avoid the heat stress/predation dilemma in any thermally stressful environment are listed.