African Red-winged Starlings prefer hexose sugar solutions, but do not like them too sweet

Abstract

Globally, two bird–pollination systems have been recognised, involving specialist and occasional nectar-feeding birds. Whether avian pollinators place selective pressure on the plants they feed on, in terms of flower morphology and nectar traits, is still debated. While considerable research exists that examines the sugar preferences, concentration preference and digestive ability of specialist nectar-feeding birds, there is little known about these parameters for occasional nectar-feeding birds, as most previous studies have not controlled for the energetic value of solutions employed in choice experiments. Birds of the Sturnidae–-Muscicapoidea lineage lack sucrase and are intolerant of solutions or fruit above 11–15% sucrose content. African Red-winged Starlings Onychognathus morio are occasional nectarivores that play a role as pollinators of several plant groups, particularly aloes, which produce large amounts of dilute floral nectar. We confirmed that these birds, like other members of their lineage studied outside of Africa, are unable to digest sucrose. In laboratory trials, they strongly preferred hexose (1:1 glucose:fructose) over sucrose solutions in paired equicaloric choice tests at a range of concentrations (5–20%) and maintained energy balance at these concentrations. However, they were unable to maintain body mass when given a choice between sucrose and hexose solutions at the highest concentration (25%) and showed no sugar preference at this level, suggesting that high concentration solutions may place physiological stress on starlings. Starlings preferentially consumed lower concentration solutions, when simultaneously offered four hexose solutions of varying concentrations (10–25%). These results suggest that starlings impose pollinator-mediated pressure for low concentration, hexose-rich nectar in the plants they pollinate.