Date of clutch initiation and mean clutch size for an alpine population of American Pipits (Anthus rubescens) in northern Wyoming varied significantly among seven years sampled during 1963–1995. Because American Pipits are obligate ground-nesters in an environment devoid of trees, I examined the hypothesis that spring snow conditions cause the annual variation in mean laying date and resulting clutch size. Mean clutch size was highly correlated with mean laying date, and both traits were highly correlated with the snow water equivalent (SWE) on 1 May. Reduced SWE resulted in earlier onset of egg laying and indirectly in increased clutch size. Trend analyses for 1961–2002 showed an 18% decline in 1 May SWE and an earlier date of snowmelt by 7 d, indicating that an earlier onset of egg laying by about 5 d may have occurred over the same period, with a corresponding increase in mean clutch size of 0.2 eggs. This result supports the global pattern for various bird species of earlier onset of nesting during the last few decades. Annual laying date and clutch size of American Pipits appear to respond predictably to short-term (annual) changes in spring snow conditions at this high-elevation location and may prove useful for projecting reproductive responses of alpine birds to changing trends in climate.
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Vol. 74 • No. 4