The impact of endophytic fungus-infected seeds on seed predators is poorly understood. In this multiple trophic level investigation, seed preference experiments were conducted to determine whether five species of passerines (dark-eyed juncos, Junco hyemalis; American tree sparrows, Spizella arborea; song sparrows, Melospiza melodia; chipping sparrows, Spizella pusilla; and house sparrows, Passer domesticus) recognize and preferentially consume noninfected (NI) over infected (I) seeds of tall fescue (Festuca arundinacea). We predicted that the birds would refrain from eating I seeds because those seeds contain high concentrations of fungal alkaloids. When given a choice of NI fescue seeds and control seeds (millet), all bird species showed a significant preference for millet. However, individuals of all species consumed some NI seeds. When given a choice of NI and I fescue seeds, all species except the chippig sparrow ate significatly more NI than I fescue seed and the chipping sparrow showed the same trend. Thus, birds were able to distinguish between the two seed types and preferred NI seeds in choice tests. Additional experiments investigated weight changes in dark-eyed juncos fed diets containing different proportions of millet, NI, and I fescue seed. Significant differences in weight loss were observed for the various diets. Juncos showed greater weight loss when the proportion of fescue seed, especially the proportion of I seed, in their diet was greater. The potential significance of the finding that abundant grass seeds are made unavailable to predators by fungal infection is discussed in relation to foraging and competition in avian communities.