The host specificity of avian lice (Phthiraptera) may be utilized by biologists to investigate the brood parasitism patterns of Brown-headed Cowbirds (Molothrus ater). As nestlings, brood parasites have a unique opportunity to encounter lice that are typically host specific. Lice are permanent hemimetabolic ectoparasites, a group found strictly on the body of the host, and they are transferred almost exclusively by bodily contact between hosts during care of young and at copulation. We investigated whether cowbird nestlings become infested with avian lice from their host parents and carry these lice away when they fledge, in effect bearing ectoparasite indicators of the species that raised them. The technique of examining the lice on cowbird fledglings to identify their foster parents would be much less costly than hiring a team of experts to determine parasitism patterns in the conventional way by finding hundreds of songbird nests. We examined 244 cowbird fledglings and found that they carried a rich fauna of lice representing 11 species and six genera, almost the entire spectrum of louse genera known to occur on passerines. We also examined 320 songbirds from 30 species, all known hosts of the Brown-headed Cowbird. As a group the host birds bore a diversity of louse species comparable to that on the fledgling cowbirds: 13 species of lice from seven genera. In contrast, most individual passerine host species yielded only 1 or 2 louse species, significantly fewer than the cowbird fledglings (p < 0.0001). Of 44 fledgling cowbirds carrying lice, 11 were linked to their probable avian foster parents via louse indicators, and these are the Wood Thrush and Red-winged Blackbird. Eighteen additional fledglings were linked to one of two possible foster parents. We concluded that cowbird fledglings do carry away host lice and this survey technique provides a partial assessment of local community parasitism patterns. The incomplete state of passerine louse taxonomy requires anyone using this technique to de-louse both cowbird fledglings and local host species in order to have a reference collection of lice. Lice from cowbird fledglings can be identified by a skilled taxonomist and linked to particular host species, and the principal difficulty is the scarcity of skilled avian louse taxonomists. We also found an unusually rich louse fauna on 219 adult cowbirds, which supports the interpretation that lack of opportunity due to physical isolation has been the fundamental factor in the host specificity of lice observed in certain avian orders.