Effects of snow-related environmental variation on breeding schedules and productivity of a high-altitude population of dusky flycatchers (Empidonax Oberholseri)

Abstract

Reproductive schedules of Dusky Flycatchers (Empidonax oberholseri) at Tioga Pass, in the central Sierra Nevada of California, varied considerably among years. Dates of earliest laying ranged from 29 May to 1 July and varied with snow depth, melt dates, and other factors associated with snowpack. A small apparent advance in laying date over the course of the study (0.52 days year-1 over 15 years) was associated with extremes in winter snowpack and spring melt that were related to wet-dry cycles in the Sierra Nevada. Spring snowpack acted on reproductive timing through direct and indirect effects of temperature, melt schedule, and phenological changes in habitat availability. The influence of these variables on laying dates was greatest on the first females to lay (most in early June) and declined through mid-July, when breeding terminated. Clutch size and fledgling production varied between years of heavy and light snowpack as a direct consequence of delayed egg laying in years of heavy snow. In years of light snowpack, females that laid eggs by mid-June produced larger clutches, fledged more young per nest, and were more likely to renest if nest failure occurred. Only two females attempted two broods in a single season, and both were among the first to breed. Seasonal declines in sensitivity to local environmental conditions may constrain the ability of Dusky Flycatchers to adjust breeding schedules to match phenological delays produced by changes in winter temperature, precipitation, and snowpack, and may limit range expansion to areas with arid winters and earlier spring melt. © The American Ornithologists' Union, 2011.