Taste Preferences, Color Preferences, and Flower Choice in Hummingbirds

Abstract

Laboratory experiments on sugar preferences and field observations on flower choice in hummingbirds [Calypte anna] indicate the following hierarchy of factors influencing preferences: energetic parameters (nectar flow of flower, concentration of feeder solution) over taste parameters (sugar composition) over color (of flower or feeder). Position factors were excluded from the experiments wherever possible because results from feeders cannot always be safely generalized to the situation in nature. Several species of hummingbirds showed the following hierarchy of sugar preferences: sucrose over glucose fructose, with equal-parts mixture of the 3 sugars falling somewhere in the middle of the preference order. The most certain result for all birds tested was the rejection of fructose, which at least in human subjects leaves a bitter aftertaste. Virtually all hummingbird-pollinated flowers studied have nectars high in sucrose. Flowers pollinated by insects, or by birds other than hummingbirds, have balanced or low-sucrose nectars. This correlation holds even among congeneric plant species having different pollinators. The production not only of abundant nectar but of high-sucrose nectar appears to be an important component in adaptation for hummingbird pollination. Some exotic flowers with low-sucrose nectars were highly preferred (but not pollinated) by hummingbirds, apparently because of their high nectar flows. Taste preferences for glucose or sucrose solutions were effective in overriding color preferences, or in establishing new color preferences. Seasonal changes in color preferences corresponding to the color of the birds'' most important food flowers suggests that color conditioning may operate in the field. Colors of hummingbird-pollinated flowers nearly always lie in the long-wavelength end of the visible spectrum. The particular color scheme shown by a plant species probably depends mainly on the visual properties of its habitat and whether the flowers are presented individually or borne in a long-lived, colorful inflorescence. The colors of tropical and temperate-zone hummingbird-pollinated flowers are similar and do not appear to be affected by the degree of migratory behavior shown by their respective hummingbirds. Spectral properties of red, orange, and yellow flowers may be fairly similar, and hummingbirds may discriminate hues better than humans at the longer wavelengths. In the context of these factors, the question "why are hummingbird flowers red?" loses much of its meaning.