WITHIN-SEASON VARIATION IN NEST NUMBERS OF DOUBLE-CRESTED CORMORANTS (PHALACROCORAX-AURITUS) ON THE GREAT-LAKES - IMPLICATIONS FOR CENSUSING

Previous censuses of cormorant colonies have been made at various stag es of the breeding season, using different census units and criteria, and little effort has been made to quantify resulting errors in popula tion estimates. The population of Double-crested Cormorants (Phalacroc orax auritus) nesting in the North American Great Lakes has been incre asing on average at 29% per annum in recent years, and there is an urg ent need for reliable population estimates, in relation to this specie s' interactions with commercial and sport fisheries. We investigated w ithin-season changes in numbers of apparently occupied nests (AONs) at 44 colony sites in lakes Superior and Huron, and at four colony sites in Lake Ontario. AON numbers increased significantly between the incu bation and nesting stages (mean 22% for lakes Superior/Huron, and 74% for Lake Ontario). The magnitude of the increase in AON numbers betwee n incubation and nestling periods was influenced most strongly by the earliness of the incubation stage census. Breeding was later at colony sites at higher latitudes, but there was little evidence of marked an nual variation in chronology. Laying at recently formed colonies was l ater than at well-established ones. We suggest that more reliable popu lation estimates for Double-crested Cormorant colonies, particularly i n rapidly expanding populations, will be obtained if future censuses a re made during the nestling stage, using our criteria for the census u nit (AON).