Variables Affecting the Density and Territory Size of Breeding Spruce‐Woods Warblers
1976
Morse, Douglass H.
Between 1969 and 1972 and during 1974 I obtained data on population density, territory size, habitat, food taken, and food available to four species of Dendroica warblers (Parulidae). Densities usually were: Black—throated Green (D. virens) > Blackburnian (D. fusca) > Yellow—rumped (D. coronata) > Magnolia (D. magnolia). Black—throated Green Warblers were commoner in red spruces than in white spruces; they showed a preference for red spruce foilage. Populations of all four species were stable between 1969 and 1972, but following two poor breeding seasons populations declined in several areas. Species with dense populations had small territories and vice versa. Territory sizes of Black—throated Green Warblers in red spruces were smaller than in white spruces. Territory size usually fluctuated even less than population density, although some territories eventually increased markedly after strong declines in abundance. The food supply usually peaked in early July. Food was more abundant in white spruces than red spruces early in June but did not consistently differ subsequently. Warblers took lepidopteran larvae in preference to other prey. The number of Dendroica species in an area was closely related to the physical characteristics of the forest, but interactions among these species mediated many of the differences in density or species composition. Mature undistributed forests in this geographic area support four of these species on a long—term basis.
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