Responses of Nesting Savannah Sparrows to Fluctuations in Grasshopper Densities in Interior Alaska

Abstract

Grasshopper populations on the Delta Agricultural Project area in interior Alaska exhibit a strong biennial periodicity. This phenomenon allowed us to study dietary, reproductive, and numerical responses of nesting Savannah Sparrows (Passerculus sandwichensis) to fluctuations in prey abundance. Grasshopper densities exceeded 25/m(2) in 1990 but were less than l/m(2) in 1991. In 1990, 61% of the identifiable food items brought to the nest were grasshoppers. In 1991, Lepidoptera larvae and Diptera were the most common items brought to the nests and no grasshopper 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 grasshopper densities in 1990. Reduced growth measurements of nestlings were 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 positive response to high grasshopper densities was observed, the magnitude of the response was diminutive compared with the large difference in prey abundance between years. This suggests that factors other than food limit reproductive output in this population of Savannah Sparrows