Attendance patterns of murres at breeding sites: Implications for monitoring

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.