Double-Crested Cormorants During the Chick-Rearing Period at a Large Colony in Southern Ontario: Analyses of Chick Diet, Feeding Rates and Foraging Directions

Abstract. During the last 25 years, North America has experienced a significant increase in Double-crested Cormorants (Phalacrocorax auritus). The increase has caused concern among wildlife and fisheries managers dealing with the perceived and/or real impacts of this bird on fisheries. Cormorant foraging and breeding ecology were examined at a large (>6,000 pairs), unmanaged colony in Lake Ontario, Canada. Chick diet, feeding rates and productivity were evaluated during 2006 and 2007. In 2007, the proportion of adult cormorants making long-range foraging trips (more than ten kilometers) during different stages of the breeding season was estimated. Alewife (Alosa pseudoharengus) composed greater than 86% of chick diet by mass during both years; suggesting that cormorants did not often prey on sport fish for feeding chicks. Chicks were fed approximately 4.5 times per day in 2006, which was significantly lower than 5.9 feeds per day during 2007. Productivity was high in both years, with 2.2 and 1.9 chicks pr...