Variations in the Distributions of Marine Birds with Water Mass in the Northern Bering Sea

Abstract

The Condor 95:33-44 C The Cooper Omitholocjcal Society 1993 VARIATIONS IN THE DISTRIBUTIONS OF MARINE BIRDS WITH WATER MASS IN THE NORTHERN BERING SEA' CHRIS s. ELPlilCK 2 AND GEORGE L. HUNT, JR. 3 Department of Ecology and El•o/utionary Biology, University of California, Irvine, CA 927 17 . Abstr~ct . We examined the pelagic distributions of 12 species of northern Bering Sea birds wtth respect to the water masses in which they were observed during the summers of 1984, 198:5 and .1986. Despite the prediction of earlier work that differences in community structure are unhkely to occur at small spatial scales, we found significant habitat preferences for all but one of the species studied. We suggest that the strength of the gradient between habitat types is the cause of this discrepancy, and that boundary conditions should also be considered when discussing the influence of spatial scales on community processes. Ker .words: Seabird distributions; Bering Sea; habitat preferences; spatial scale; boundary co11d11 ions. INTRODUCTION Patterns in the distribution and abundance of seabirds at sea vary as a function of the spatial and temporal scales at which they occur (Hunt and Schneider 198 7). According to these authors, macroscale ( 1,000-3,000 km; after Haury et al. 1978) patterns of seabird distribution most likely reflect variations in primary and secondary pro- duction, whereas rnesoscale patterns (I 00-1 ,000 km) involve variations in avian species com- position in response to variations in the com- position of prey communities. At still smaller scales (1-100 km). the abundances of individual species often reflect opportunities to forage at local concentrations of prey (Hunt and Schneider 1987). In this generalized scheme, variations in the avian community are alternately character- ized by changes in either biomass or species com- position, and reflect a similar alternation in the prey community. Despite the prediction, based on the above scheme, that differences in the species compo- sition of seabird communities are unlikely to ex- ist over relatively small spatial scales, at least two lines of evidence have indicated that such patterns exist. First, Haney ( 1986) has shown significant differences in the densities of several bird species over four water masses associated with Gulf Stream eddies while working at the ' Received 13 March 1992. Accepted 9 September 2 Present address: Program in Ecology, Evolution, and .Conservation Biology, 1000 Valley Road, Uni- versity of Nevada, Reno, NV 89512. 1 Corresponding author. lower end of the mesoscale range (50-150 km). Second, in the Chirikov Basin of the northern Bering Sea, where three separate water masses occur in close proximity, investigations of the distribution of nesting colonies of marine birds have shown that colonies located in Alaska Coastal Water support primarily piscivorous species, whereas colonies near Anadyr Water are dominated by planktivorous auklets (Sowls et aJ. 1978, Springer and Roseneau 1985). These patterns indicate that differences exist in the use of different water masses by different species (Springer ct al. 1987). This supposition was strengthened by limited transect data (Drury et al. 198 1; summarized by Hunt et al. 1981) that showed considerable differences in use of the eastern and western portions of the Chirikov Ba- sin by birds. This paper investigates the foraging distribu- tions of seabirds in the Chirikov Basin with re- spect to water masses and their constituent prey communities. We test whether a variety of spe- cies show habitat specificity when several habi- tats occur over a small area (on the order of the birds' daily flight ranges). The study, however, was conducted at a spatial scale generally asso- ciated with responses to changes in local prey concentrations (Schneider 1982, Schneider and Piatt 1986, Schneider et al. 1987), rather than to different prey communities. In addition to our findings for the Chirikov Basin, we discuss the effect of the boundary gradient between habitat patches on a bird's ability to respond to habitat changes and the need to examine the combined effects of boundary conditions and scale on hab- itat selection by birds.