Interpreting habitat quality requires an understanding of the consequences of habitat use because bird density alone may not reflect habitat quality. Although habitat use can be readily quantified, subsequent effects remain elusive for most species, especially for migrants on the wintering grounds. Wintering Henslow's Sparrow (Ammodramus henslowii) densities in Longleaf Pine (Pinus palustris) savannas are fire-dependent, with highest bird densities in the first winter after a fire, but reduced density in subsequent winters without fire. We determined habitat characteristics associated with variation in bird density in the first and second winters following fire and then followed birds' diet, condition, and survival in 10 plots over two winters. Habitat structure and seed energy availability, but not plant species composition, were equally important for explaining bird density, except for two outlier plots with extremely high seed production and fewer birds than expected. Henslow's Sparrows consumed a variety of seeds and arthropods, which indicates that they are generalists capable of responding to fire-mediated changes in seed composition. Body condition decreased in midwinter, associated with a decrease of arthropods in the birds' diet. Although Henslow's Sparrow densities responded strongly to ephemeral savanna conditions, consequences of habitat use were negligible because survival and three measures of body condition did not vary with years since fire or among age and sex classes. These aspects of the winter ecology of sexually monochromatic Henslow's Sparrows are consistent with tenets of the ideal free distribution, whereby differences in bird density between habitats result in equal fitness.
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Vol. 128 • No. 3