Blowflies belong to the genus Protocalliphora . In North America, Protocalliphora occur in Alaska, throughout Canada, in most of the contiguous United States, and in parts of Mexico. If you find evidence of blowflies in your nest boxes, it's likely that more than one species is present. Blowflies spend the winter as adults outside the nest box, burrowed in leaf litter, crevices in tree bark, or some other protected site. In the spring, female blowflies deposit their eggs in nests containing very young nestlings.
Some species deposit their eggs directly on the nestlings.
The egg stage is short, probably 24 to 48 hours. This makes sense, as parasite eggs laid in the nests of birds with short nestling periods must hatch quickly to complete their development before the young birds leave the nest. The larval period of the blowfly varies between 7 and 15 days, depending on temperature, the host bird's species, and the species of fly. Timing is everything, for the parasite eggs must hatch and the larvae must feed during a relatively short period. During nest-box checks the larvae can sometimes be seen attached to the fleshy parts of the chick's head, legs, or underwings.