SCHONDUBE, J.E.*: Nectar digestion by flowerpiercers and hummingbirds

Flowerpiercers and hummingbirds are the two most specialized groups of nectar feeding birds in the neotropics. Because species of both groups are syntopic and can even be found feeding on the same plants, they provide a unique opportunity to study how different taxa cope with a nectar diet. I studied the nectar digesting traits of the Cinnamon-bellied Flowerpiercer (Diglossa baritula) and compared them with those of a syntopic hummingbird of the same body mass (Eugenes fulgens). Both species have the same gut length, and gut nominal area. Contrasting with other passerines that do not digest sucrose, flowerpiercers, as hummingbirds, feed intensely on floral nectars that are rich in this sugar. Diglossa assimilates sucrose with the same efficiency as hummingbirds (98-99%). But its sucrase activity levels are 2 to 5 times lower than those of hummingbirds. The functional morphology of Diglossa�s gut seems to compensate for its lower sucrase activity. When taking into account the tri-dimensional structure of the gut, Diglossa presents a larger surface area than hummingbirds. The presence of special invaginations, called plica, suggests that Diglossa captures nectar in its intestine, amplifying its enzymatic digestion by increasing mean retention time. Eugenes has a simpler gut, in which digesta seems to flow faster and sucrose is digested completely by high levels of enzymatic activity. Differences in transit times and mean retention times among these species support this hypothesis. There is a relation between the ability to digest sucrose and the sugar selection patterns of these two species that seems to affect their foraging decisions in the field.