The Importance of Survey Timing in Monitoring Breeding Seabird Numbers

Abstract

-We conducted weekly aerial surveys of islands along the central Maine coast from April-June of 19931997 and used aerial photographs to determine peak nest count dates for Double-crested Cormorants (Phalacrocorax auritus), Herring Gulls (Larus argentatus) and Great Black-backed Gulls (Larus marinus). These data also were used to determine the potential effect of survey timing on the ability to detect long-term trends in the abundance of these species. The number of cormorant nests in the study area peaked in mid-June, while Great Black-backed and Herring gulls peaked in late May and earlyJune, respectively. Peak nesting dates generally were consistent for each island across years, but varied by up to a month between islands during a given year. A 10-year monitoring program using annual surveys conducted between 23 May and 23June, or biennial surveys conducted from 2-17June, would have an 80% probability of detecting annual changes of ?5% for all three species in this region. Received 1 November 2000, accepted 4 December 2000.