Overlap in Foods and Foraging of Four Species of Blackbirds in the Potholes of Central Washington

Overlap in foods and foraging of four species of breeding blackbirds in the Potholes of central Washington was estimated from samples of food delivered to the nestings obtained by the pipe—cleaner technique. Prey distribution and availability were sampled with emergence traps for aquatic species and by sweeping with a standard insect net for upland species. Overlap was estimated by the probability than an item of food drawn randomly from the diet of one species would be of the same prey category taken from the same habitat as an item drawn at random from the diet of another species. Because of the diurnal cycle of emergence of aquatic insects, overlaps were calculated for three periods of the day (preemergence, emergence and postemergence) and averaged for the final overlap value. The overall food overlap for three of the species is high for all periods of the day and there is little evidence for differential specialization within habitats. Habitat overlaps, however, are varied, being highest during the emergence period for Redwings and Yellowheads but highest during the afternoon between Redwings and Brewers. The total overlap values are very similar to the theoretical maximum values derived from competition theory by MacArthur and Levins.