Colony Attendance of Least Auklets at St. Paul Island, Alaska: Implications for Population Monitoring

Abstract

Colony attendance by Least Auklets (Aethia pusilla) was monitored during three breeding seasons at St. Paul Island, Alaska. Maximum counts of birds attending one 150-m2 study plot varied significantly from year to year, with a nearly two-fold difference between highest and lowest years. Maximum numbers on the surface at any one time amounted to about half of the local breeding population. Attendance was high in the year with greatest proportion of adults breeding, low in the year with lowest proportion of adults breeding, and the proportion of adults breeding differed significantly among years. Adult attendance both early and late in breeding season differed significantly among years, but there was no evidence that varying attendance related to changes in the overall adult population. Counts during incubation and chick-rearing stages were affected by sub-adults (twoyear-olds), which differed significantly in attendance from year to year and sometimes represented up to half the birds on the colony surface. Year-to-year changes in surface counts probably related to strength of this sub-adult cohort and to varying attendance behavior of adults and sub-adults that correlated with food availability. These data suggest that, taken alone, surface counts at Least Auklet colonies must be interpreted cautiously in assessing population changes. Suggestions for improvement of counting techniques and an alternative approach to population monitoring are discussed.