Intake responses in nectar feeding birds: Digestive and metabolic causes, osmoregulatory consequences, and coevolutionary effects

Nectar-feeding vertebrates respond to variation in nectar sugar content by modulating volumetric intake. In some nectar feeding animals, the intake response to sugar concentration can be accurately predicted from simple mathematical models that rely on knowledge of gut morphology, in vitro rates of sugar digestion, and daily energy expenditures. Because most of the floral nectars consumed by vertebrates are dilute, these animals ingest large amounts of water while feeding. The water turnover rates of hummingbirds feeding on dilute nectar are more similar to those of amphibious and aquatic organisms than to those of terrestrial vertebrates. Dilute nectars can pose osmoregulatory challenges for nectarivores. Nectarivorous birds exhibit renal traits that are well suited to dispose of large water loads and that appear inadequate to produce concentrated urine. Nectar-feeding birds prefer concentrated over dilute sugar solutions. However, the concentration difference that they can discriminate is smaller at low than at high concentration. We hypothesize that this pattern is a consequence of the functional form of intake responses that often results in decelerating sugar intakes with increasing sugar concentration. The diminishing returns in floral attractivity that may result from increased nectar concentration may be one of the reasons why the nectars of hummingbird pollinated flowers are dilute in spite of the preference of birds for higher concentrations. The intake responses of nectar-feeding birds capture the integration of a behavioral response with the physiological processes that shape it. Because the behavior of nectar-feeding birds can have consequences for the plants that they visit, the intake response may also have coevolutionary effects.