We described the community composition of the ectoparasitic flies Protocalliphora Hough (Diptera: Calliphoridae) and their Nasonia (Ashmead) (Hymenoptera: Pteromalidae) parasitoids collected from tree swallow, Tachycineta bicolor (Vieillot), nests in southern Québec, Canada, in 2008 and 2009. The prevalence of nest infestation by Protocalliphora was 70.8% in 2008 and 34.6% in 2009. The average parasitic burden of Protocalliphora spp. was estimated at 5.53 (±5.61 SD) pupae per nestling for 2008 and 4.66 (±9.31 SD) pupae per nestling for 2009. The percentage of nests containing Protocalliphora pupae parasitized by Nasonia spp. was of 85.3% in 2008 and 67.2% in 2009. Three species of Protocalliphora were collected—Protocalliphora sialia Shannon & Dobroscky, Protocalliphora bennetti Whitworth, and Protocalliphora metallica (Townsend)—and two species of Nasonia, Nasonia vitripennis (Walker) and Nasonia giraulti Darling. This is the first record of P. bennetti and N. giraulti in the province of Québec. Our findings provide further evidence for observation made previously that altricial bird nests are more frequently and more heavily infested by blow flies in western than in eastern North America. Our data also suggest that more than a quarter of the tree swallow within the study area are exposed to high levels of ectoparasitism that could negatively affect their health.