Hummingbird Licking Behavior and the Energetics of Nectar Feeding

--Key assumptions and components of biophysical models of hummingbird licking that are central to current interpretations of plant-pollinator coevolution have not previously been verified. To test the realism of the models, I measured fine-scale parameters of a Rufous Hummingbird's (Selasphorus rufus) licking, including licking frequency and volume extracted per lick, with a photodetector array that monitored movement of the tongue and the nectar-pool meniscus. Both licking frequency and volume per lick decreased with increasing concentration, contradicting previous suggestions that hummingbirds may maintain constant licking frequency or volume per lick at all concentrations. At three nectar-pool volumes, energy-intake rates were significantly higher at 25 to 35% sucrose than at higher concentrations, supporting the models' qualitative prediction that optimal nectar concentration is low when energy-intake rate is averaged over the time scale of licking. Received 21 October 1993, accepted I April 1994. THE ADAPTIVE SIGNIFICANCE of low nectar concentrations prevalent in hummingbird-pollinated flowers has attracted much attention (Baker 1975, Bolten and Feinsinger 1978, Calder 1979, Pyke and Waser 1981, Plowright 1987). Mean nectar concentrations of North American and many tropical hummingbird-pollinated plants average 20 to 25% sucrose equivalents (mass/total mass; Baker 1975, Pyke and Waser 1981, Heyneman 1983). In choice tests, however, hummingbirds have preferred concentrations higher than 45% (Van Riper 1958, Hainsworth and Wolf 1976, Stiles 1976, Pyke and Waser 1981, Tamm and Gass 1986). This preference for concentrations roughly twice what flowers offer in nature has complicated understanding of the presumably coevolved plantpollinator system (Pyke and Waser 1981, Feinsinger 1987, Gass 1988). Optimal nectar concentration for hummingbirds should be predictable from the details of their feeding behavior and fluid-dynamical properties of nectar. Hummingbirds feed by licking with their forked, open-grooved tongues, into which nectar flows by capillary action during the loading phase when the tongue contacts the nectar pool (Weymouth et al. 1964, Hainsworth 1973, Ewald and Williams • Present address: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA. 1982). Increasing concentration increases the nectar's caloric value, but also increases its viscosity and, therefore, decreases its volumetricflow rate into the tongue grooves. The combination of these effects causes energy-intake rate to peak at an intermediate concentration (Heyneman 1983, Kingsolver and Daniel 1983). Biophysical models (Kingsolver and Daniel 1983) suggested that the sucrose concentration that maximizes birds' energy-intake rates is 20 to 25% for low nectar-pool volumes that can be loaded on single licks, but 35 to 40% for high volumes that require many licks to empty. The model prediction that optimal concentration depends on nectar-pool volume seemed to resolve the discrepancy between birds' observed preference for high nectar concentrations and the low concentrations many flowers provide them. All published choice tests used feeders containing volumes that were essentially infinite from the birds' perspective, and for which the model predicted high optimal concentration. In contrast, most flowers visited by nontraplining hummingbirds usually contain nectar pools near to or less than the volume of the tongue grooves (which is 1.9 •1 for Rufous Hummingbirds [Selasphorus rufus]; unpubl. data; pool volumes for several plant species are summarized in Gass and Roberts 1992). Under these conditions, when the nectar pool could be emptied on a single lick, the model predicted low optimal concentration. The predicted upward shift in optimal con-