Brood-parasitic nestlings have a unique opportunity to encounter host-specific lice (Phthiraptera). Lice are permanent ectoparasites found strictly on the body of the host, and they are transferred almost exclusively by bodily contact during copulation and care of young. We investigated whether Brown-headed Cowbird (Molothrus ater) nestlings become infested with lice from their host parents and carry these after fledging, in effect bearing ectoparasite indicators of the species that raised them. Examining lice on cowbirds to identify foster parents would be less costly than determining parasitism patterns in the conventional way by finding many host nests. The 244 cowbird fledglings that we examined carried 11 species and 6 genera of lice, almost the entire spectrum of louse genera known from passerines. We also examined 320 songbirds from 30 species of hosts. As a group, the diversity of louse species on hosts was comparable to that on fledgling cowbirds: 13 species and 7 genera. In contrast, most individual host species yielded only one or two louse species, significantly fewer than on cowbird fledglings. Of 44 fledgling cowbirds with lice, 11 were linked with probable avian foster parents, and 18 other fledglings were linked with one of two possible foster parents. We conclude that cowbird fledglings carry away host lice and that our technique provides a partial assessment of parasitism patterns. The incomplete state of louse taxonomy requires that users of the technique obtain a reference collection of lice from host species in addition to the sample collection from cowbird fledglings. Lice from cowbird fledglings can be identified by a taxonomist and linked to particular host species, and the principal difficulty is the scarcity of skilled 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 in certain avian orders.