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THE CO N DOR JOURNAL OF T HE COOPER ORNITHO LOGICA L SOCIETY Number 3 Volume 83 August 1981 Condor 83: 193-203 © The Cooper Ornithological Society 1981 THERMAL STRESS AND PREDATION: INFLUENCES ON THE STRUCTURE OF A GULL COLONY AND POSSIBLY ON BREEDING DISTRIBUTIONS J. L. HAND G. L. HUNT, JR. AND M.WARNER ABSTRACT.-The arrangement of nesting te rritories of Lams 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 fo r 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 is lands, which may be related to heat stress and associated problems of pre- dation 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 e nvironment are listed. In appearance and behavior (Hand 1979), Two of the three recognized subspecies of Western Gul l (Lams occidentalis) closely all three subspecies are kin to other large, resemble each other. They breed on off- white-headed gulls of both hemispheres shore islands along the west coast of the (Moynihan 1959), typified by the H erring United States and Baja California. The third Gull (L. argentatus) in the northern hemi- subspecies, L. a. livens, differs from the oth- sphere and the Kelp Gull (L. dominicanus) er two in a number of characteristics, one of in the southern hemisphere. Most gulls which is colony structure-the subject of breed at latitudes above 30°, in boreal, sub- this investigation. L. o. livens is virtually antarctic, and arctic habitats. L. o. livens, in restricted to the Gulf of California (approx- contrast, breeds on desert islands under ex- imately 23-32°N, 113°W; Devillers et al. tremely hot and dry conditions. 1971) and appears to be fully isolated geo- A striking feature of its colony structure graphically from the Pacific Coast subspe- is that nests are nearl y always placed on cies (Devillers et al. 1971, Hand 1979). Be- beaches or low-l ying peninsulas, usually in cause th is su bspecies differs in several a roughly linear manner slightly above the respects from the other two (reviewed in high-tide line. These gull s may also nest in Hand 1979), several investigators have sug- semi-isolation: a single pair may be the only gested that it should be recognized as a sep- gulls occupying the beach of a small cove. arate species (Dickey and van Rossem 1925, Thus, the territories of most pairs are on Devillers 1971, LeValley 1975, Hand, in the waterfront and their owners have un- press). impeded access to the water without having ll931

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