Floral constancy in bumble bees: handling efficiency or perceptual conditioning?

Individual bees often prefer flowers of the same species that they are already foraging on, and other individual bees prefer other flowers. This "floral constancy" has classically been explained as a learned behavior by which bees avoid wasting time switching between handling techniques. Choice trails were given to Bombus vagans workers that were freely foraging in mixed and pure fields of Trifolium pratense, T. repens, Viccia cracca, and Prunella vulgaris. Contrary to expectation, (1) bees showed if anything a stronger preference for their flower type in pure fields where they lacked experience than in a mixed field where they had had the opportunity to learn, (2) there was greater constancy in a mixed field of the two morphologically similar Trifolium species than in a mixed field of the morphologically disparate T. pratense and P. vulgaris, and (3) bees were more willing to switch between flowers of distinct morphologies when the colors were similar than between flowers of distinct colors when the morphologies were similar. We suggest that constancy is due to some form of perceptual conditioning whereby individual bees become temporarily sensitized to one or a few floral cues.