Spatial Variation in Winter Survival of Song Sparrows Melospiza melodia

Abstract

The resident population of Song Sparrows breeding on Mandarte Island, British Columbia fluctuated markedly between 1975 and 1989. A major population crash occurred during a week of unusually severe winter weather in early February 1989. Over 90% of Song Sparrows alive on the island in mid-January 1989 disappeared by 9-12 February 1989. The high mortality was accompanied by directional natural selection favoring female birds with longer wings. No selection was detected among males. We addressed the alternative hypothesis that Song Sparrows died of disease rather than poor weather. The population of Fox Sparrows Passerella iliaca wintering on Mandarte also declined sharply during the same period as Song Sparrows. The contemporaneous decline in the population size of these two species supports the conclusion that severe weather caused the population crashes. However, Fox Sparrows survived the poor weather better than Song Sparrows, possibly because of greater fat reserves in the former species. The effect of cold weather on population size of Song Sparrows was local. Sexspecific adult mortality was highest on Mandarte, lowest at a nearby mainland site provided with supplemental winter food, and intermediate on similar islands near Mandarte.