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The timing of breeding in double-crested cormorants (phalacrocorax auritus albociliatus) : its effects on clutch size, nestling growth, diet and survival Sullivan, Terrance Michael


I examined the effects of a delayed time of breeding on clutch size, nestling growth and diet of Double-crested Cormorants (Phalacrocorax auritus albociliatus) at three colonies, Five Finger Island, Mandarte Island and the Fraser River, during the 1993 and 1994 breeding seasons. The effect of population changes was also monitored at these colonies and 4 additional colonies, Crofton, Chain Island, Christie Islet and Hudson Rocks, all situated within the Strait of Georgia As the breeding season progressed, asymptotic mass and condition index of the nestlings declined. In general, nestlings produced early in the season were heavier, had a slower growth rate and required more time to achieve their final mass than those produced later in the season. The last birds to breed were on the Fraser River (1993) where successful egg laying occurred approximately 3 months after the first colony (Five Finger Island) bred. The patterns of growth of the structural components differed at this colony from all others. The asymptotic length of both the culmen and tarsus, and the overall size of the nestlings increased as the season progressed. These parameters were also larger than at all other colonies. Nestling cormorants on Five Finger Island were fed mainly- Pacific Sandlance, nestlings on Mandarte Island were fed mainly blennies and those on the Fraser River were fed mainly Pacific Staghorn Sculpin and Shiner Perch. Colony differences probably reflect colony locations in the Strait of Georgia but may be influenced by the time of breeding. The quantity or quality of food delivered to the nestlings did not differ among colonies, except for the amount of lipids. Differences in nestling growth could not be explained by differences in the diet. To determine the effect of the timing of breeding on the changes in cormorant numbers, I modeled the dynamics of the population. Both the predicted and observed changes in the breeding population followed the same trend: at colonies where breeding occurred early in the season, there was an overall increase in the breeding populations while those that bred later in the season showed a decline. This trend was hypothesized to result from poor survival of the nestlings resulting in low recruitment into the breeding population.

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