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

Publisher Website
Google Scholar
Abstract
Mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) migrate to and breed in montane areas that sustain large variations in snowfall. The encounter considerable annual variation in environmental conditions upon arrival at their breeding grounds. At Tioga Pass [California, USA], breeding is delayed in years of deep snowpack. Individual pairs will occupy patches of habitat as soon as they are snow-free. Under such conditions, many females alter an apparent preference for nesting in dense grass on the ground, and nest in small pine trees, willows and elsewhere. Once the birds are ready to breed and habitat is available, nesting proceeds relatively unaffected by temperature extremes or storms. In years of heavy snow, some habitat is unavailable until late summer. This reduces the number of breeding pairs in a given area and may decrease breeding success or productivity for the population. The birds renest readily but usually cease breeding after a brood is fledged. Suitable breeding habitat is distributed in patches at the end of the season wherever surface water continues to sustain wet-green conditions. In such areas efforts at renesting are prolonged and double-brooding may occur. Schedule of arrival, modes of foraging, management of energy reserves, flexibility in nest placement, renesting capabilities and ability to withstand brief spells of bad weather permit successful reproduction in subalpine areas despite large annual variations in environmental conditions.