Comparison of Pellets versus Collected Birds for Sampling Diets of Double-Crested Cormorants

For many fish-eating birds, two types of samples can be used to determine diets of adults during both breeding and nonbreeding periods: esophagus and gizzard contents of birds collected at feeding sites, and pellets (indigestible residue) cast up at roosts or breeding colonies. We compared these two methods for Double-crested Cormorants (Phalacrocorax auritus) collected on the North Platte River, Wyoming and nesting at a nearby colony both before and after fingerling trout were stocked in the river. Before stocking, there were no significant differences between pellets and collected birds in percent masses of different fish types in different length classes. After stocking, the two methods yielded similar results for different length classes, but differed in relative proportions of suckers and trout. Patterns in percent occurrence of crayfish in the diet suggest that the time of day adults are collected might affect comparability of the two methods.