We tested the nestling discrimination hypothesis by observing responses of female Red-winged Blackbirds (Agelaius phoeniceus) to playback of begging calls by conspecifics or Brown-headed Cowbirds (Molothrus ater). Female redwings departed the nest to forage and returned with food significantly faster in response to redwing playback than to cowbird playback. We also observed feeding rates at redwing nests before and after we removed either a redwing or cowbird nestling. Female redwing feeding rates changed significantly with these manipulations, but there was no difference in response to redwing versus cowbird nestling removal. Although the removal experiment shows that parasitized and unparasitized broods receive equal care from their hosts, the playback results are consistent with the hypothesis that female Red-winged Blackbirds detect a difference between begging calls of conspecifics and parasites.