Forest edges are often associated with a high risk of brood parasitism by Brown-headed Cowbirds (Molothrus ater), but the extent to which this vulnerability changes across the breeding season is poorly understood. We evaluated temporal variability in the relationship between distance to habitat edges and risk of brood parasitism. From 2008–2010, we monitored parasitism of 347 nests of Northern Cardinals (Cardinalis cardinalis) at 19 forested sites within the urbanizing landscapes of central Ohio. For each nest, we measured distance to the nearest edge (e.g., road, residential area, mowed field, agriculture, and natural area) and recorded when clutches were initiated. We found that risk of parasitism associated with edges varied with time of season. Early in the breeding season, risk of parasitism declined with increasing distance from edges. However, late in the breeding season, nests farther from edges had higher rates of parasitism. These patterns suggest that early in the season, cowbirds search for forest nests along residential and agricultural edges, whereas later in the season, cowbirds search the interior. Our results show that managers should consider temporal factors in generalizing about the risk that edges pose to nests.