Spruce budworm outbreaks and the incidence of vagrancy in eastern North American wood-warblers

The Tennessee (Vermivora peregrina), Cape May (Dendroica tigrina), Bay-breasted (D. castanea), and Canada (Wilsonia canadensis) warblers and the Ovenbird (Seiurus aurocapillus) appear to show positive numerical responses to outbreaks of the spruce budworm (Choristoneura fumiferana); the Magnolia (D. magnolia), Black-throated Green (D. virens), and Blackburnian (D. fusca) warblers may occur in lower numbers during outbreaks because of increased competition with these budworm specialists. Thus, we predicted that the number of fall vagrant Tennessee, Cape May, and Bay-breasted warblers and Ovenbirds occurring in California are highly positively associated with budworm population density, positively intercorrelated, and negatively associated with numbers of Magnolia, Black-throated Green, and Blackburnian warblers. A 23-year (1972-1994) data set of budworm population densities and vagrant warbler occurrences in California showed that (i) budworm population density was an excellent predictor (explaining about 50% of the variance) of numbers of occurrences of the Cape May, Bay-breasted, and Canada warblers; (ii) all warbler species tended to co-occur, with numbers of Cape May and Bay-breasted warblers especially highly intercorrelated; and (iii) Magnolia Warbler numbers were negatively associated with those of budworm specialists, but Black-throated Green and Blackburnian warblers showed no association.