FORAGING ABILITY OF RUFOUS HUMMINGBIRDS ON HUMMINGBIRD FLOWERS AND HAWKMOTH FLOWERS

We examine the suitability of ornithophilous flowers and sphingophilous flowers in Ipompsis and Aquilegia for nectar foraging by the hummingbird Selasphorus rufus. In S. rufus, bill length averages 18.9 mm in females and 17.3 mm in males. Maximal tongue extension approximates bill length, suggesting that birds can feed from floral tubes up to 33.5 mm in length. However, their ability to do so is limited by two factors. First, the maximal depth at which S. rufus can extract nectar decreases with the width of the floral tube. Second, feeding time is shortest in short floral tubes and progressively increases as the tubes lengthen because of increased time required for tongue extension and retraction. Hence, nectar foraging occurs with optimal efficiency in moderately broad floral tubes with lengths that do not exceed or only slightly exceed the bill length plus less-than-or-equal-to 5-mm tongue extension. In most ornithophilous taxa of Ipomopsis and Aquilegia, the floral tubes have lengths and widths in the optimal range for nectar foraging by S. rufus, whereas in the sphingophilous taxa, the basal nectar either may be reached but at much higher cost or, more frequently, is beyond reach of bills and tongues. The flower-visiting habits of S. rufus and other hummingbirds in nature are generally congruent with these conclusions and support the case for coadaptation between these plants and pollinators.