Intake Responses in Nectar Feeding Birds: Digestive and Metabolic Causes, Osmoregulatory Consequences, and Coevolutionary Effects

Nectar-feeding vertebrates respond to variation in nectar sugarcontent by modulating volumetric intake. In some nectar feedinganimals, the intake response to sugar concentration can be accuratelypredicted from simple mathematical models that rely on knowledgeof gut morphology, in vitro rates of sugar digestion, and dailyenergy expenditures. Because most of the floral nectars consumedby vertebrates are dilute, these animals ingest large amountsof water while feeding. The water turnover rates of hummingbirdsfeeding on dilute nectar are more similar to those of amphibiousand aquatic organisms than to those of terrestrial vertebrates.Dilute nectars can pose osmoregulatory challenges for nectarivores.Nectarivorous birds exhibit renal traits that are well suitedto dispose of large water loads and that appear inadequate toproduce concentrated urine. Nectar-feeding birds prefer concentratedover dilute sugar solutions. However, the concentration differencethat they can discriminate is smaller at low than at high concentration.We hypothesize that this pattern is a consequence of the functionalform of intake responses that often results in deceleratingsugar intakes with increasing sugar concentration. The diminishingreturns in floral attractivity that may result from increasednectar concentration may be one of the reasons why the nectarsof hummingbird pollinated flowers are dilute in spite of thepreference of birds for higher concentrations. The intake responsesof nectar-feeding birds capture the integration of a behavioralresponse with the physiological processes that shape it. Becausethe behavior of nectar-feeding birds can have consequences forthe plants that they visit, the intake response may also havecoevolutionary effects.