Hummingbird sweetness preferences: taste or viscosity?

Previous studies on hummingbirds have attempted to predict the preferred percentage of sucrose in solution for optimal energy uptake. These values range from 22% to 40%. This study compared the Archilochus alexanderi (black-chinned hummingbird) behavioral response to nectars based on sucrose receptors, sweetness receptors, and by a physical measure of viscosity (thickness/stickiness). The study was conducted in Santa Cruz county, Arizona, and data was collected from ten feeders each with three red, plastic flowers with small “corolla tubes” allowing hummingbird access to the nectar inside. A 20% sucrose solution (average natural flower sucrose concentration) was used as the control nectar, and test nectars were introduced periodically. Test nectar variables included varying levels of sucrose in solution, artificial sweeteners (saccharin, Equal, and aspartame) and carboxy methyl cellulose (used to increase viscosity). The study found that the black-chinned hummingbird did not consume significantly different amounts of 30% sucrose solutions than 20% but that they showed a significant preference for 40% sucrose solution (almost four times that of the control). The study also found that both artificial sweeteners and increased viscosity neither increased nor deterred visitation. In addition, the researchers observed that hummingbirds responded to decreased sucrose concentrations by increasing their sampling behavior at feeders. It was concluded that chemosensory mechanisms, as opposed to physical measures of viscosity, are used by hummingbirds when evaluating and selecting sucrose nectars. The results also support an existing model that predicts that even with added feeding costs associated with higher nectar viscosities, higher sucrose concentrations are preferred by hummingbirds.

Stromberg, M. R., and P. B. Johnsen. 1990. Hummingbird sweetness preferences: taste or viscosity? The Cooper Ornithological Society 92: 606-612.