Cowbird Parasitism in the Kansas Tallgrass Prairie

Abstract

During 1974-1975 brood parasitism by the brown-headed cowbird (Molothrus ater) was studied in a tallgrass prairie community [consisting of Andropogon scoparius, A. gerardi, Sorghastrum nutans, Panicum virgatum, Bouteloua curtipendula and Poa pratensis] in northeastern Kansas [USA]. Hosts of primary interest were the grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana) and eastern meadowlark (Sturnella magna), although ancillary data are recorded for the cardinal (Cardinalis cardinalis), blue grosbeak (Guiraca caerulea), red-winged blackbird (Agelaius phoeniceus), robin (Turdus migratorius), brown thrasher (Toxostoma rufum) and orchard oriole (Icterus spurius). A total of 77 nests of the 3 ground-nesting species were found, of which 54 (70.1%) were parasitized. Parasitized nests contained an average of 2.7 cowbird eggs/nest. In parasitized nests of 5 spp., 13 (8.3%) of 157 cowbird eggs were successful; 17 (12.4%) of 137 host eggs were successful. In the unparasitized nests, 23 (21.5%) of 107 eggs were successful. A high intensity of brood parasitism was indicative of high relative densities of cowbirds. In this system nesting success was very low, suggesting that the high densities of cowbirds were due to high adult survival. The implications of this situation are discussed [with the use of models].