Attendance Patterns of Murres at Breeding Sites: Implications for Monitoring

Abstract

We studied attendance patterns (presence and absence at breeding sites) of common murres (Uria aalge) and thick-billed murres (U. lomvia) at a colony in the Semidi Islands, western Gulf of Alaska. The period from mid-laying through hatching was appropriate for censusing because counts were subject to the least amount of daily variation. Time of day and weather did not have large effects on the counts and serial dependence was weak or absent. A trend in murre numbers during the census period occurred in 2 of 4 years but the direction of change was inconsistent. Counts made on 40 consecutive days were needed to detect a 6-8% change in numbers between years, whereas 5 counts were adequate to detect a 20-26% change in numbers. Annual changes in murre numbers varied among study plots but mean combined counts differed by 6-16% between years. Behavioral differences alone were inadequate to explain the annual variation in plot counts because site occupancy rates of individual murres were relatively constant. J. WILDL. MANAGE. 53(2):483-493 Common murres and thick-billed murres are abundant and widely distributed seabirds in temperate to high latitude regions of the northern hemisphere. Their colonies are often large (>10,000 birds), but the species are vulnerable to human disturbance, pollution, drowning in fish nets, and competition with commercial fisheries (Vermeer 1976, Nettleship 1977, Birkhead and Nettleship 1980). Consequently, there has been considerable effort to develop techniques and establish long-term programs for monitoring populations of murres (e.g., Wanless et al. 1982, Murphy et al. 1986, Rothery et al. 1988). Murres often occur in dense aggregations of breeding and nonbreeding birds, and breeding pairs do not build nests. Censuses are usually based on repeated counts of individuals, but variation within seasons can obscure the annual differences that are of primary interest for population monitoring. Furthermore, annual differences in colony attendance may arise from factors other than a change in population size. Our objective was to examine temporal and spatial components of variation in colony attendance at an Alaskan colony, the effects of weather, and site occupancy by murres of known breeding status. We apply the results to questions about sampling design (timing, location, and no. counts) and to the interpretation of apparent changes in population size. We thank L. A. Terwilliger and M. R. North for help with the field work in 1986, and G. V. Byrd, D. K. Cairns, D. Heinemann, P. D. Martin, D. N. Nettleship, and J. F. Piatt for critically reading the manuscript. Field work was funded in part by the Alaskan Outer Continental Shelf Environmental Assessment Program (OCSEAP), administered by the Bureau of Land Management and National Oceanic and Atmospheric Administration. STUDY AREA AND METHODS The study was conducted on the Semidi Islands (560N, 1560W), a remote archipelago of 9 islands in the western Gulf of Alaska. An estimated 1.1 million murres bred on the islands (Hatch and Hatch 1983). The primary area we observed was a 2-km expanse of 50-200-m cliffs on the west side of Chowiet Island, the largest island in the group (1,300 ha). Birkhead and Nettleship (1980) defined 2 possible approaches to monitoring populations of murres. Type I methods entail frequent, timeconsuming observations on study plots throughout the breeding cycle, from which the number of breeding pairs/plot and fate of all eggs and hatchlings is known. Type II methods involve replicate counts of the number of birds/plot and provide an index of population size over the period of observation. A Type II approach is always less time-consuming than a Type I study, but it provides no information on the breeding status or activity patterns of individual birds that 'Present address: Department of Biological Sciences, University of Alaska, Anchorage, AK 99508.