We analyzed 55 years of abundance data (1960–2014) for Sitta canadensis (Red-breasted Nuthatch) to seek patterns of winter irruptions on temporal and spatial scales. This species shows an erratic pattern of irruption into southerly areas from its northern breeding areas. Irruptions show a broad geographic synchrony. At the narrower level of the state or province and even more so at the level of individual counts, correlations of abundance in adjacent areas become weaker. The abundance of irruptive birds is best considered a mosaic. At the regional scale, correlations of Red-breasted Nuthatch abundance with irruptive northern finches that also depend on conifer seeds are weak to absent. The data suggest that birds irrupt because of failure of conifer seed production on the breeding grounds, not because the birds are seeking masting conifer stands to the south.
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Vol. 24 • No. sp7