EFFECT OF SEXUAL DIMORPHISM IN BILL LENGTH ON FORAGING BEHAVIOR - AN EXPERIMENTAL-ANALYSIS OF HUMMINGBIRDS

We examined whether sexual differences in trophic morphology are assoc iated with sexual differences in foraging behavior through two laborat ory experiments on rufous hummingbirds (Selasphorus rufus) designed to compare probing abilities (maximum extraction depths) and handling ti mes of sexes at flowers. Bills of female S. rufus are about 10.5% long er than bills of males, and this difference was associated with sexual differences in foraging abilities. Maximum extraction depths of femal e S. rufus were significantly greater than those of males, and no over lap between the sexes was observed. Moreover, handling times of female s were shorter than handling times of males at flowers having longer c orollas (greater-than-or-equal-to 15 mm). Thus, because of their longe r bills, female S. rufus have the potential to feed from longer flower s than males, and can do so more quickly. We suggest that no single me chanism is responsible for the evolution of sexual dimorphism in bill lengths of hummingbirds, but rather that the dimorphism probably refle cts the combined effects of reproductive role division and intersexual food competition, and possibly, sexual selection.