Habitat Distributions of Wintering Sparrows: Foraging Success in a Transplant Experiment

Why do birds partition habitats? Possible explanations include nonoverlapping food distributions, interspecific competition, and predation. If food limits distributions, species should forage most successfully in their preferred habitats and should experience poorer food intake rates elsewhere. These same predictions hold if species distributions are shaped by exploitative competition. We tested the foraging success hypothesis and exploitative competition hypothesis as explanations of the habitat distributions of Sage Sparrows (Amphispiza belli), Black—throated Sparrows (A. bilineata), and Dark—eyed Juncos (Junco hyemalis), which spend the winter in different habitats along an elevational gradient in the Sonoran Desert of southern California. Individuals of each species were transplanted between habitats in a portable aviary and observed while foraging on naturally occurring seeds. Predictions of the foraging success hypothesis and exploitative competition hypothesis were not confirmed. Only Dark—eyed Juncos achieved their highest foraging success in their preferred habitat. Black—throated Sparrows experienced nearly identical foraging success in all three habitats, yet are confined to only one of them naturally. Sage Sparrows foraged more successfully in habitats other than that in which they naturally occur. We conclude that immediate foraging gains do not explain habitat distributions of these birds, and that interference competition, predation, or other unknown factors are responsible. We contrast this finding with that seen in Galapagos finches in which bird distributions closely matched food supply.