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1. Sage sparrows [Amphispiza belli (Cassin)], black-throated sparrows [A. bilineata (Cassin)] and dark-eyed juncos [Junco hyemalis (L.)] winter in different habitats along an elevational gradient in the Sonoran Desert of southern California, USA. Such species replacements along environmental gradients are commonly attributed to interspecific competition. We tested the alternative hypotheses that food, predation and microhabitat structure might shape species distributions.

2. Species abundances were unrelated to food availability. Species were missing from habitats in which food standing crop was no less abundant than in the habitats they occupied. A second measure of food availability, predicted food intake rate, also failed to explain the distributions. The profitabilities of seed species ranked similarly among the three species of sparrows suggesting that the sparrows should prefer the same foods and should have similar habitat distributions if habitat distributions were shaped by food.

3. All three species escape predators by fleeing to woody cover and all prefer to feed near cover. Hence, if predation risk determined sparrow distributions, all species should be most abundant in the habitat with the most escape cover; this was not observed.

4. Foraging microhabitats used by individual species were more widely distributed than the species themselves, suggesting that species’ distributions are not limited by microhabitat structure. Also, although species occupy different habitats, they overlap extensively in the types of microhabitats that they occupy 5. Although predictions of the competition hypothesis were not tested, species distributions and the results above are consistent with the hypothesis that interspecific competition is responsible for habitat partitioning by the species. Direct tests of this hypothesis are warranted.