We collected observational data in three consecutive breeding seasons to study interactions between the botfly Philornis seguyi and Red-crested Cardinals (Paroaria coronata) in a temperate zone near the southern limit of Philornis distribution. We analyzed: (1) seasonal trends in prevalence of parasitism, (2) influence of botfly parasitism on nestling growth rate and survival, and (3) the association between nest site vegetation at different scales (i.e., nest tree, vegetation surrounding the nest tree, and landscape) and probability of botfly parasitism. Prevalence of parasitism was 28% and was higher later in the breeding season. Botfly parasitism produced sub-lethal (lower growth rate of nestlings that survive) and lethal (lower nestling survival) effects. The lethal effect was negatively associated with age at the time nestlings were parasitized. Botfly parasitism was not associated with vegetation characteristics at the level of nesting tree or vegetation surrounding the nesting tree, but was associated with landscape features. Parasite prevalence was higher in large continuous woodland patches than in small isolated patches. However, we did not observe increased use of isolated patches of forest by Red-crested Cardinals, suggesting that use of nest sites with high botfly parasite intensity could be the consequence of high host density.