Pollinator availability as a determinant of flowering time in ocotillo (Fouquieria splendens).

Ocotillo, a perennial shrub of Sonoran and Chihuahuan Deserts, produces its red tubular flowers in spring. This timing coincides with northward migration of hummingbirds through desert areas. Observations of visitors, pollen collections, and seed set reductions following exclusion of different flower visitors indicate that both hummingbirds and solitary bees pollinate ocotillo in southern Arizona. Seed sets of flowers on marked plants varied considerably within and between years, and this variation was related to the temporal match between flowering and hummingbird migration. This suggests that selection acts on plants to synchronize flowering with periods of pollinator abundance.