EFFECTS OF HABITAT AND INVERTEBRATE DENSITY ON ABUNDANCE AND FORAGINGBEHAVIOR OF BROWN-HEADED COWBIRDS

We Studied foraging brown-headed Cowbirds (Molothrus ater) in central Missouri to determine the influence of habitat type and invertebrate b iomass on cowbird abundance and behavior. We measured flock size, dens ity, peck rate, foraging time, vigilance, aggression, and invertebrate abundance in five habitats. Seven sites contained shore-grazed grass, short-ungrazed grass, tall-grazed grass, tall-ungrazed grass, and fee dlot habitat treatments. Cowbird flock sizes were largest in short-gra zed grass, but densities were highest in feedlots. Foraging time and a ggression did not differ among habitats, but peck rates were highest i n 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 vigilan t 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. Cow bird flock size was positively related to invertebrate density, but fo raging 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 becau se of readily available grain and selected short-grazed grass because of the availability of invertebrate foods. Compared with grazing, gras s height was of secondary importance to foraging cowbirds.