Larval Protocalliphora Hough are obligate hematophagous parasites of nestling birds. While there is much interest in their effects on birds, little is known about the factors influencing their population sizes within nests. We experimentally increased temperature at nests of tree swallows (Tachycineta bicolor Vieillot) on two study areas and compared densities of larval Protocalliphora with control nests. We found that number of Protocalliphora varied in a curvilinear fashion with temperature, with population sizes highest in nests around 25°C and decreasing at both higher and lower temperatures. After controlling for temperature, we found differences between study areas in number of Protocalliphora in nests, and we hypothesize that these results are caused by differences in the size and dispersion of nests between the two areas. There was no seasonal variation in number of Protocalliphora in nests, but numbers increased significantly with the number of hosts in a nest, presumably because more resources were available for exploitation as host brood size increased. The cell-mediated immune responses of nestling tree swallows was positively correlated with number of Protocalliphora, and we suggest that these results are the consequence of the immune systems of nestlings in heavily parasitized nests being more sensitive to challenges by novel antigens or mitogens. To our knowledge, our study is the first to show variation in densities of larval Protocalliphora as a consequence of experimental manipulation of temperature and provides insight into important factors affecting population sizes of parasites in avian nests.