Snow Conditions and the Onset of Breeding in the Mountain White-Crowned Sparrow

Abstract

Since the summer of 1968, my students and I have been studying the breeding biology of the Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha) in the Mono Basin of the Sierra Nevada of California, especially at Tioga Pass. This bird summers in high mountain meadows of western North America and winters in the southwestern United States and northern Mexico. Earlier we reported on reproductive schedule and success of oriantha during the consecutive seasons of 1968 through 1970 (Morton et al. 1972). Of all environmental factors affecting the reproductive efforts of this bird, snow conditions seem to be most important. In many years snowpack seems to determine arrival and occupancy schedules at breeding grounds. Duration of snowpack and rate of disappearance affect the availability and suitability of habitat throughout much of the breeding season. The 1969 season of our study followed a winter with the heaviest snowpack ever recorded in the Mono Basin and the 1976 season followed a winter with one of the lightest snowpacks recorded. Herein I compare data from these seasons, and others, in order to provide insight as to how migratory birds are adapted for breeding under the range of conditions found in high mountains. Most of our study was conducted at Tioga Pass Meadow, a 0.5 x 1.0 km subalpine meadow that extends northward from Tioga Pass (elev. 9,500-10,300 ft).