Summer and Migrational Movements of Satellite-Marked Doublecrested Cormorants from a Breeding Colony Managed by Egg-Oiling in Lake Ontario, USA

A two-year satellite telemetry study was initiated in May 2000 at a Double-crested Cormorant (Phalacrocorax auritus) breeding colony on Little Galloo Island (LGI) in eastern Lake Ontario, New York, USA, which is managed by egg-oiling. The objective was to describe cormorant (N = 26/year) movements, specifically during the period of reproductive management by egg-oiling and seasonally (breeding, migration and wintering). Egg-oiling at two-week intervals resulted in a hatch success on LGI of 5.7% for 2000 and 2001, combined. The majority (97%) of core use areas of marked cormorants contained LGI throughout three egg-oiling treatments (six weeks), and 71% still contained LGI by the end of the final (fourth) treatment (eight weeks). Of cormorants that moved during or after control activities, three remained in the vicinity of active breeding colonies for over three months. Cormorants initiated fall migration over a 16-week period ranging from 12 July to 29 October, with a mean departure date of 6 September (N = 24, SE = 8 days) over both years. Mean duration of fall migration was 34 days (N = 19, SE = 7 days, range = 108 days). Most (75%) cormorants captured at LGI migrated east of the Appalachian Mountains, and their winter range extended from southeastern Louisiana, along the coast of the Gulf of Mexico, to the southern portion of the Atlantic coast. Although three (13%) cormorants over both years relocated to other active colonies for long enough periods (over three months) to potentially raise young, this study indicates that control efforts did not result in complete abandonment of LGI. Egg-oiling was successful in reducing recruitment within breeding seasons, and within-breeding-season renesting attempts by cormorants in this study were limited and likely unsuccessful. Further evaluation and refinement of egg-oiling as a management tool will require multiyear monitoring of the LGI cormorant breeding colony