Effects of blow fly (Protocalliphora spatulata: Diptera: Calliphoridae) parasitism on the growth of nestling savannah sparrows in Alaska

We investigated the effect of blow fly larvae (Protocalliphora spatulata) on nestling savannah sparrows (Passerculus sandwichensis) in Alaska subject to biennially fluctuating prey abundance. Grasshopper populations in some regions of interior Alaska can show a strong biennial periodicity. Growth of nestling savannah sparrows, numbers of blow fly larvae in nests and on nestlings, and grasshopper populations were monitored during four breeding seasons (1990–1993) on the Delta Agriculture Project site near Delta Junction, Alaska. Growth parameters for parasitized and nonparasitized nestlings were compared within and between years. There were no significant differences in growth parameters between parasitized and nonparasitized nestlings between any years. There were also no differences in growth parameters within years, except for tarsus length in 1991 and nestling mass in 1993 (low grasshopper years); in 1993, parasitized nestlings were heavier and the sample size was small. Additionally, there were no differences in fledging success within years. Although parasitism combined with other factors (e.g., food limitation) may have an impact on avian reproduction, our data do not support this.