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, short-ungrazed 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. Received 12 March 1997, accepted 3 September 1997. ALTHOUGH BREEDING ECOLOGY, host interactions, and parasitism levels have been widely studied in Brown-headed Cowbirds (Molothrus ater), little is known about their feeding ecology (see Lowther 1993). Historically, Brown-headed Cowbirds may have been limited to the shortgrass plains because of their feeding requirements. The cowbird's life history likely evolved on the Great Plains with bison (Bison bison). Bison ate and trampled grass in a manner that may have provided suitable vegetative structure for foraging cowbirds, exposed foods such as insects and seeds, and possibly increased the availability of insects to cowbirds (Friedmann 1929). Cowbirds commonly feed in shortgrass habitats such as pasture and other agricultural habitats such as feedlots, horse corrals, and croplands (Friedmann 1929, Mayfield 1965, Rothstein et al. 1980, 1984, Thompson 1994, Thompson and Dijak 1998). 3Present address: U.S. Forest Service, North Central Forest Experiment Station, 1-26 Agriculture Building, University of Missouri, Columbia, Missouri 65211, USA. E-mail: dmorris@cmc2.cmc.edu Mechanisms underlying the association of feeding cowbirds with livestock and shortgrass habitats have not been determined. Cowbirds could benefit directly from the presence of cattle because cattle expose invertebrates and may serve as perches or protective cover. Cowbirds could benefit indirectly from cattle because grazing maintains shortgrass cover. Cowbirds might prefer shortgrass cover because of increased visibility to detect predators and conspecifics, or because of more efficient movement or foraging. Shortgrass habitats also might offer a high density food because grazed habitats often have high numbers of invertebrates (Coyner 1938, Holmes et al. 1979, Capinera and Sechrist 1982, Jepson-Innes and Bock 1989). Despite detailed observations on foraging cowbirds (Friedmann 1929, Mayfield 1965, Williams 1975), there has been little analysis of cowbird foraging behavior or feeding-habitat preference. Thompson and Dijak (1998) showed that feeding cowbirds preferred grassland and feedlots, but they did not differentiate between grazed and ungrazed or tallgrass and shortgrass habitats.