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

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 produced per nest in 2006 and 2007, respectively. The decrease in productivity corresponded with an increase in colony size, from 6,125 to 7,241 pairs. In 2007, approximately one-fifth of the nesting colony foraged at least ten kilometers throughout the breeding season. Cormorants fed their chicks very few sport fish in the Toronto area; however, more work is required to determine whether the consumption of Alewife by cormorants represents competition with sport fish for prey.