A survey of museum specimens revealed large interspecific variability in the sex ratio in 14 species [Mew, ring-billed, laughing, Franklin's, Bonaparte's, Heerman's, California, herring, Thayer's, western, glaucous, glaucous-winged, and great black-backed gulls, and blacklegged kittiwake] of North American gulls. In the three black-headed species, (Larus atricilla, L. pipixcan, and L. philadelphia), females outnumbered males among prefledglings, but males outnumbered females among adults. In the white-headed gulls, the male/female ratio in prefledglings did not vary significantly from 1.0 in any species. However, among first-year birds, females outnumbered males in 8 white-headed species. Among adults, the male/female ratio was < 1.0 in the Mew and Ring-billed gulls, > 1.0 in the Great Black-backed Gull, and = 1.0 in the other white-headed gulls.

Average clutch size for large Larus gulls is close to three eggs, and the production of a clutch of four is uncommon (Keith 1966; Paludan 1951; Vermeer 1963). We report here on a colony of Western Gulls (Larms occidentalis) in which many clutches containing four and five eggs were found. It is of particular interest that in these large clutches not only was hatching success low but also eggshell thickness was reduced.

The southeastern Bering Sea is characterized by three mixing regimes, separated by fronts associated with the 50, 100, and 200 m isobaths. Phytoplankton to zooplankton transfer-rates are high in waters over the outer shelf and slope (seaward of the 100 m front) relative to transfer in waters over the middle shelf (between the 50 and 100 m fronts). To see whether this difference is reflected at a higher trophic level, we computed carbon flux to the 11 commonest seabird species. Bird-density data (for the period 1975 through 1979) were combined with daily caloric requirement, which is an allometric function of body size in this endothermic group. Minimum transfer to seabirds over a 153 d period (April-August) was 30 mg C m-2 for the middle shelf and 48 mg C m-2 for outer shelf and slope waters. Trophic transfer to subsurface-feeding birds (shearwaters, murres and auklets) differed little between regions. In contrast, trophic transfer to surface-feeding birds (fulmars, petrels, and kittiwakes) in the outer shelf and slope waters was 3 times greater than in the waters of the middle shelf. Thus, for seabirds as a whole, pathways of energy transfer differed more between regions than did total carbon flux. © 1982 Springer-Verlag.

With the study of the equilibrium theory of island biogeography pioneered by MacArthur and Wilson ( 1963, 1967), increasing attention has been paid to the study of turnover rates (Mayr 1965; Diamond 1969, 1971; Power 1972; Terborgh and Faaberg 1973). The failure of bird species turnover rates to show the predicted negative correlation with island size in the California Channel Islands (Diamond 1969) raises questions as to what factors other than island size may be involved in determining species turnover. · Previous studies of avifaunal turnover have treated faunas as a whole and have not differentiated turnover rates for rare species from those of common species. Since very large birds and species at high levels on the food chain often exist at relatively low densities, these organisms would be expected to have a greater probability of extinction on small islands than those species which maintain larger populations (Brown 1971). A theoretical basis for this hypothesis can be derived from Gilpin ( 197 4). He has shown ( pers. comm.) that extinction rates should be a negative exponential function of K, and thus should be very sensitive to small population size.

We investigated the foraging distribution of 2 species of murres (Uria lomvia and U. aalge) in relation to Acoustically Determined Biomass (ADB) and hydrographic structure surrounding the Pribilof Islands, Bering Sea, Alaska, USA. The distribution and abundance of both murre species were similar; therefore, we combined data for thick-billed and common murres in the analysis. We found that murre densities were higher in frontal regions than in non frontal regions. Maximum ADB was also found to be elevated in the frontal zone. Within frontal regions, high densities of murres were associated with regions containing relatively high levels of ADB. In contrast, we did not find significant correlations between murre density and ADB in the study area as a whole. We postulate that murres restrict their foraging to the frontal region because prey can be located more predictably within this zone than in other areas in the vicinity of the Pribilof Islands.