Field evidence for resource competition between species comes mostly from studies of the effects of competition (Schoener 1983), with little attention paid to the behavioral processes of interspecific competition (Morse 1970, Myrberg and Thresher 1974). Interspecific territoriality has been cited as evidence of interference competition in a number of studies (reviewed by Orians and Willson 1964; Murray 1971, 1981). Evidence for interspecific aggression without territoriality in birds is provided by Livezey and Humphrey (1985), Mumme and de Queiroz (1985), and Nuechterlein and Storer (1985), but none of these studies demonstrated a competitive function. Murray (1981) has suggested that some cases of interspecific aggression may result from mistaken identity. Payne and Groschupf (1984) concluded that interspecific call-site defense by two species of indigobirds, Vidua funerea and V. raricola, is a result of mistaken identity resulting in courtship disruptions rather than competition for food. Here I present data that demonstrate that male Pin-tailed Whydahs, V macroura, selectively pursue and supplant granivorous species from feeding areas on their territory. Pin-tailed Whydahs are small (about 14 g), polygynous, brood-parasitic grassland finches in which the males are strongly interspecifically territorial during the breeding season (Collias cited in Friedmann 1960, Shaw 1984, pers. observ.). Whydahs are well-known by aviculturalists, as well as field biologists, for being very pugnacious toward other species as well, chasing all manner of birds, both small and large (e.g., Harman and Vriends 1978, Shaw 1984). Whydahs feed almost exclusively on the ground by picking up fallen grass seeds (Friedmann 1960, Fry 1975), although occasionally they will pick Paspalum scrobiculatum seeds directly off the plant while standing on the ground (pers. observ.). For this reason they prefer disturbed grasslands with patches of bare soil where they can feed.