<p> The bushtit (Psaltriparus minimus) of Southeastern Arizona is a cooperative breeder, in which 35% of nests have more than two birds attending. Helpers. join nests at all stages of the breeding season, and potentially make genetic contributions to the nest. Behavioural observations suggest that the fluid social system of the bushtit may provide opportunities for extra-pair fertilizations (EPFs) and intra-specific brood parasitism (ISBP). DNA fingerprinting was applied to assess parentage in the 1988 and 1992 breeding seasons. Using three minisatellite probes (Jeffreys 33.15, 33.6 and PER), fingerprints were generated from nine complete families (nestlings and adults) from 1992 and twelve families of nestlings, in this study, from 1988. Parentage analysis of the 1992 families indicated strict genetic monogamy, despite the fact that two of the breeding groups were socially polyandrous. A case of serial monogamy in a double-brooded nest provided evidence of a male helper achieving reproductive success after aiding in the rearing of non-kin in the first brood. This result suggests the potential reproductive benefits of helping behaviour in a double-brooding species. Genetic relatedness, within groups of nestlings from the same brood, was analyzed in the 1988 samples. In using the 1992 data as a calibration, the 1988 broods were found to contain no unrelated dyads resulting from ISBP. Discrimination between full and half­ sibling relationships was less clear. The analysis of relatedness serves as a useful comparison to other studies that have used a multiple probe approach to assess relatedness. </p> / Thesis / Master of Science (MSc)