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Female bees are usually confronted with a choice among several flower species that differ in their location and abundance within the community, and in the efficiency with which their pollen and nectar can be harvested. We investigated the effects of distance and flower density of two flower species on pollen collection by providing nest locations for the mason bee Osmia lignaria in natural settings. Distance weakly affected pollen use; on average, bees nesting near a flower species tended to collect more of its pollen than did bees nesting at a greater distance. Flower density did not predictably impact pollen use, and use did not track changes in density during the season. Bees consistently mixed pollen from more distant species, despite substantial added foraging costs, and also mixed when one species was an order of magnitude less abundant than the other. Bees require nectar as well as pollen to feed their offspring, and our preliminary data suggest that the efficiencies of pollen and nectar collection are inversely related between the two flower species, which would favor visitation to both species. Bees appear to collect some pollen from the low-pollen, high-nectar plant while visiting it for nectar. Thus, a nectar-collecting constraint may favor collecting pollen from mixtures of species. [ABSTRACT FROM AUTHOR]