Penstemon species of western North America show various degrees of apparent adaptation to different pollinators, especially hummingbirds and bees. In the Chiricahua Mountains, Arizona, we studied 3 species that are visited by hummingbirds but differ in apparent degree of floral specialization, as well as in habitat. Two montane species, P. barbatus and P. pinifolius, have narrow tubular corollas colored red to scarlet, whereas P. pseudospectabilis of the foothills has a wider purple corolla. We predicted (a) that hummingbird visits would be more important to the former two species than to P. pseudospectabilis (as measured by seed set through a plant's maternal function), and (b) that nectar rewards would be higher in the hummingbird-specialized species. All 3 species were regularly visited by hummingbirds and small halictid bees; nectar was the primarily reward sought by halictids as well as by hummingbirds. P. pseudospectabilis was also heavily visited by honey bees, which entered the wide corolla tube easily. Honey bees were present at the montane site, but were excluded from nectar by the narrow tube of P. pinifolius and rarely entered the somewhat larger tube of P.barbatus. In caging experiments, seed set of flowers exposed to bees but not hummingbirds was substantially higher than that of flowers in a "no pollinators" treatment, in all 3 species. Flowers exposed to hummingbirds as well as bees had greater seed set than those in the bees-only treatment, but this effect was more pronounced in the two more specialized species (4-5 fold increase) than in P. pseudospectabilis (2-fold increase). These results supported our first prediction. The second hypothesis was not supported, as the net daily nectar production rate of P. pseudospectabilis was the highest among species measured.

Key words: Penstemon<\I> Scrophulariaceae<\I> hummingbirds<\I> pollination<\I>