RESPONSES OF NESTING SAVANNA SPARROWS TO FLUCTUATIONS IN GRASSHOPPER DENSITIES IN INTERIOR ALASKA

Grasshopper populations on the Delta Agricultural Project area in inte rior Alaska exhibit a strong biennial periodicity. This phenomenon all owed us to study dietary, reproductive, and numerical responses of nes ting Savannah Sparrows (Passerculus sandwichensis) to fluctuations in prey abundance. Grasshopper densities exceeded 25/m(2) in 1990 but wer e less than l/m(2) in 1991. In 1990, 61% of the identifiable food item s brought to the nest were grasshoppers. In 1991, Lepidoptera larvae a nd Diptera were the most common items brought to the nests and no gras shopper deliveries were observed. Pledging success was 97% in 1990 and 96% in 1991. At least one nestling per nest successfully fledged each year. Mean clutch size was significantly higher during the high grass hopper densities in 1990. Reduced growth measurements of nestlings wer e obtained in 1991 compared to 1990. Effects on growth appeared to be greater early in development, rather than prior to fledging. Savannah Sparrow densities did not differ between the two years. Although a pos itive response to high grasshopper densities was observed, the magnitu de of the response was diminutive compared with the large difference i n prey abundance between years. This suggests that factors other than food limit reproductive output in this population of Savannah Sparrows .