DIET AND PREY USE PATTERNS OF THE AMERICAN WHITE PELICAN (PELECANUS-ERYTHRORHYNCHOS) NESTING AT PATHFINDER-RESERVOIR, WYOMING

We investigated the diet of the American White Pelican (Pelecanus erythrorhynchos) breeding on Bird Island, Pathfinder Reservoir, Wyoming, in 1986, 1988 and 1989, examining 584 regurgitations representing 11,717 prey items. Twenty-four prey species were found in the regurgitated boli. More than 90% of the numerical composition consisted of the Iowa darter (Etheostoma exile), crayfish (Orconectes spp.), tiger salamander (Ambystoma trigrinum), fathead minnow (Pimephales promelas) and white sucker (Catostomus commersoni). Over 83% of the biomass consumed was composed of common carp ( Cyprinus carpio), white suckers and tiger salamanders. We found year-to-year variation for numbers of fathead minnows, crayfish and tiger salamanders taken by white pelicans. Differences in numbers of white suckers, common carp, Iowa darters and tiger salamanders consumed by pelicans occurred within the breeding period during 1988. In 1989, we found within-breeding season differences in numbers of common carp and crayfish consumed by pelicans. Annual and nesting season dietary switches were more probably related to changes in food availability than to chick age-dependent prey selection. It appears that prey vulnerability primarily determined the composition of the white pelican diet at Pathfinder Reservoir. White pelicans mostly consumed the susceptible bottom-dwelling species in water less than 1 m deep or prey within 1 m of the surface of deep water.