The Summer Biology of an Unstable Insular Population of White-Crowned Sparrows in Oregon

Abstract

We analyzed nest productivity, population size, and temporal aspects of nesting, molting, and premigratory fat deposition in an isolated population of Mountain Whitecrowned Sparrows (Zonotrichia leucophrys oriantha) on the eastern slope of Hart Mountain, Oregon, from 1972 to 1986. This population, nesting from 1,800 to 2,300 m, is near the lower altitudinal limit of oriantha's geographic distribution at the latitude of southern Oregon. Hart Mountain is an isolated massif separated by unsuitable habitat from nearby populations of this bird in the Steens Mountains (80 km) and the Warner Mountains (65 km). The sex ratio of adults was probably 1:1. The number of pairs in a defined sample area diminished from 41 in 1974 to 16 in 1979, and to a smaller but unmeasured number by 1982 to 1986. Losses of eggs and nestlings to predators and to snowstorms were comparatively very large, and productivity (0.99 to 1.72 fledged young per pair per year) was inadequate to replace losses of adults. The year-to-year return ratio (minimum survivorship) of adults was likewise comparatively low, 0.37 in males, 0.31 in females, indicating that mortality was unusually large or that breeding-area fidelity was poor. Analysis of return ratios in relation to population size showed that new oriantha were recruited to the study population from other localities, but not in sufficient numbers to maintain stability. Summer time budgets of oriantha on Hart Mountain were very similar to time budgets of two other populations of these birds in the subalpine zone, suggesting that the Hart Mountain population was not stressed by shortages of time. The postnuptial and postjuvenal molts, in fact, were longer than those of oriantha in a subalpine population. We suggest that we witnessed in 1972 to 1986 the declining phase of repeated cycling of numbers, or of extinction and refounding, that may characterize insular populations or populations at their limits of distribution. On Hart Mountain, heavy losses of progeny to predators and weather were not being compensated by recruitment of adults from nearby sources.