Effects of Habitat and Invertebrate Density on Abundance and Foraging Behavior of Brown-Headed Cowbirds

Abstract

We studied foraging Brown-headed Cowbirds (Molothrus ater) in central Missouri to determine the influence of habitat type and invertebrate biomass on cowbird abundance and behavior. We measured flock size, density, peck rate, foraging time, vigilance, aggression, and invertebrate abundance in five habitats. Seven sites contained short-grazed grass, shortungrazed grass, tall-grazed grass, tall-ungrazed grass, and feedlot habitat treatments. Cowbird flock sizes were largest in short-grazed grass, but densities were highest in feedlots. Foraging time and aggression did not differ among habitats, but peck rates were highest in feedlots, and vigilance at foraging sites was highest in short-grass habitats, Females spent more time foraging than males, but peck rates did not vary significantly with sex. Males spent more time in vigilant and aggressive behaviors than did females. Invertebrate biomass and density were lowest in feedlots, Large flock sizes and high peck rates coincided with high invertebrate densities in short-grazed grass. Cowbird flock size was positively related to invertebrate density, but foraging time and peck rate were not related to invertebrate density. We conclude that cattle were an important component of the habitat that influenced cowbird foraging behavior. Cowbirds selected feedlots because of readily available grain and selected short-grazed grass because of the availability of invertebrate foods. Compared with grazing, grass height was of secondary importance to foraging cow­birds.