Bird blowflies, Protocalliphora spp. (Diptera: Calliphoridae), are prevalent ectoparasites of altricial bird nestlings across the Holarctic region. Yet, their spatial and temporal dynamics of infestations, species composition, and interactions with parasitoids remain poorly understood. We present a 16-year (2004–2019) multisite study of bird blowfly infestations based on 2673 tree swallow, Tachycineta bicolor (Vieillot) (Passeriformes: Hirundinidae), nests collected across a 10 200-km² gradient of agricultural intensity in Quebec, Canada. Nest infestation prevalence and parasitic load varied markedly across space and time but showed synchronous recurrence at approximately 75% of sites, suggesting the influence of regional and local processes. Yearly rates of parasitoidism of bird blowfly puparia by Nasonia spp. wasps (Hymenoptera: Pteromalidae) were high but variable (48–90%), likely contributing to the temporal fluctuations in bird blowfly prevalence and load. Substantial interannual shifts in the relative abundance of Protocalliphora species ( P. bennetti , P. metallica , and P. sialia ) emphasised the importance of species-level identification in bird blowfly ecological studies. Large overlap in puparia size among species challenged the utility of traditional diagnostic traits for species identification. Finally, dormancy or mortality of Nasonia spp. occurred in 3–16% of Protocalliphora spp. puparia, depending on year. These findings highlight the importance of long-term, multitrophic, and spatially explicit monitoring to unravel the drivers of host–parasite–parasitoid dynamics.