The preferences that bird species exhibit for different sugar types may play a role in pollinator-mediated selection on plant nectar composition. How bird sugar-type preferences are affected by concentration is a fundamental question for research that aims to use those preferences to explain plant nectar composition. Such research has been done for several specialist avian nectarivores, and for a few passerine occasional nectarivores, but not, apparently, for any nonpasserine occasional nectarivore. We examined the effect of concentration of sugar solutions on sugar-type preferences of the Speckled Mousebird (Colius striatus), a common African nonpasserine that occasionally feeds on floral nectar. Surprisingly, the species exhibited a concentration-dependent switch in sugar preference, from hexose sugars at low concentration to sucrose at high concentrations, that was remarkably similar to that shown by specialist avian nectarivores and that contrasts with results reported for passerine occasional nectarivores. During these experiments, Speckled Mousebirds were unable to meet energy demands on low-concentration diets and lost significant body mass when fed 5% and 10% solutions. The birds showed no preference among four different hexose solutions representing the range of concentrations (10–25%) found in bird-pollinated flowers, and there was no difference in their ability to digest sucrose and hexose sugars. Our results suggest that Speckled Mousebirds have sugar-preference traits that are similar to those of specialist avian nectarivores, whereas their apparent lack of concentration preference is similar to that found in passerine occasional nectarivores.
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Vol. 127 • No. 4