Status, Distribution and Habitat Affinities of Double-Crested Cormorant Nesting Colonies in Wyoming

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Abstract
Double-crested Cormorants (Phalacrocorax auritus) were reported breeding in Wyoming [USA] in 1928, when one active nest was located on Yellowstone Lake. In 1983 the Double-crested Cormorant breeding population consisted of 629 pairs in 9 colonies, and it had increased to 1477 nesting pairs in 18 colonies by 1986. The marked increase of breeding population is associated with the construction of irrigation reservoirs. Seventy-three percent of the Double-crested Cormorant colonies in 1986 were on man-made water impoundments. Reservoirs provide isolated islands or flooded trees for nesting substrate. They also probably provide foraging habitat. Although most recent colonies were in flooded vegetation, significantly more cormortants nested on the ground. In spite of recent population increases in Wyoming, it seems unlikely that this trend will continue. Nesting habitat is currently threatened by decay of nest trees and deterioration or loss of nesting islands brought on by excessively high or low water levels. Current breeding populations need to be protected from deterioration of nesting habitat and human disturbance of breeding birds. It will be necessary for natural resource management agencies to implement appropriate management strategies to maintain viable nesting populations of Double-crested Cromorants in Wyoming.

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