Flies in the genus Protocalliphora are important nest parasites of chestnut-backed and mountain chickadees in El Dorado County and Modoc County, California. An undescribed Protocalliphora species was the predominant parasite at each site. Infestation rates of chickadee nests in nestboxes exceeded 95% in El Dorado County and 92% in Modoc County; a maximum of 273 larvae were recovered from a single nest. In contrast, only one of 84 chickadee nests was infested in Contra Costa County nestboxes.

The Protocalliphora species studied was univoltine, and both sexes overwintered as adults. Dissections of related species suggest multiparity with a maximum ovipositional potential of 90 eggs per gonotrophic cycle. Eggs were placed in the nest material, and females readily oviposited in previously parasitized nests. Oviposition occurred only when nestlings were present and without discrimination with regard to nestling age. Duration of the egg stage was less than 48 hours. The first instar lasted 1 to 2 days, the second instar 2 to 3 days, the third instar 7 to 10 days, the prepupa 2 to 3 days, and the pupa 10 to 36 days.

The undescribed Protocalliphora species was specific to cavity-nesting birds in forest habitats. The location and acceptance of potential hosts by various Protocalliphora species appear to be governed by two different sets of stimuli. Protocalliphora asiovora Shannon and Dobroscky and P. sialia Shannon and Dobroscky were attracted to, but never reared from, chickadee nests. Nest odor probably aids Protocalliphora females in nest location, but other short-range stimuli apparently determine which hosts are acceptable for oviposition.

Nest infestation levels of the undescribed Protocalliphora species were not related to differences in nest site environment, but they were related to nest size. Larger nests apparently provided the larvae with refuges from probing activities of adult chickadees, allowing maximum survival in these nests. Natural enemies did not appear to play a major role in the population dynamics of this Protocalliphora species.