Dilute bird nectars: viscosity constrains food intake by licking in a sunbird

Abstract

Floral nectars of bird-pollinated plants are relatively dilute. One hypothesis proposed to explain this concerns the difficulty for birds of drinking nectar of high viscosity. We examined the effects of viscosity, separately from those of sugar concentration, on feeding by captive whitebellied sunbirds (Cinnyris talatala). Viscosities of artificial nectar (sucrose solutions ranging in concentration from 0.25 to 1.5 mol/l) were altered with Tylose, an inert polysaccharide. Food consumption was measured over 3 h, and lick frequency and duration were recorded using photodetection devices on feeding apertures too small for the bill but large enough for the extended tongue. Volumetric intake rates (ml/s) were inversely proportional to nectar viscosity, and were similar over the range of sucrose concentrations when viscosity was held constant. Sucrose intake rates (mg/s) remained the same on pure sucrose solutions, but they decreased with increasing viscosity at a constant sucrose concentration. Lick frequencies and tongue loads were reduced at high viscosities, and lick duration increased, which confirms that sunbirds take longer to ingest viscous solutions. Licking behavior was remarkably similar in birds feeding on different sucrose concentrations if viscosity was held constant. Nectar ingestion rate is determined by viscosity; however, total food intake is mainly modulated by sugar concentration. Similar effects of food viscosity have been observed in insects that suck nectar.

The hummingbird tongue is a fluid trap, not a capillary tube

“…Our observation of rapid lamellar unfurling rules out the idea that the hummingbird tongue tip acts as a set of static capillary tubes during nectar feeding (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)41). The tongue does not passively draw floral nectar up into the grooves via capillarity when its tips contact the liquid; rather, it is dynamically trapping nectar within the lamellae while the tips leave the fluid.…”

“…Thus, it is reasonable to assume, on the basis of the anatomical evidence, that the dynamic nectar-trapping mechanism documented here is present in every species of hummingbird. We suggest that dynamic nectar trapping is likely to be a component of the feeding mechanics of other nectarivorous birds with convergent tongue morphologies (26,28,41,49,50). Mechanistically, dynamic trapping appears likely to be functionally superior to simple capillarity in two ways: (I) the tongue-loading rate is not limited by the nectar displacement inside the tongue grooves (which makes it potentially faster) and, perhaps more importantly, (ii) the tongue tip can capture fluid successfully (filling its entire capacity) even in thin layers of nectar.…”

“…Since then, all models of foraging strategy (including those predicting concentration preferences) and energy balance in hummingbirds have calculated calorie intake rate under the assumption that the tongue tip is in the form of a pair of semicylindrical grooves that fill passively via capillarity when in contact with the nectar (21)(22)(23)(24)(25)(26). The notion that fluid is drawn toward the mouth from the tongue tip and along the lingual grooves through the action of capillarity is currently widely accepted (19)(20)(21)(22)(23)(24)(25)(26)(27)(28). However, if capillarity were responsible for tongue loading, the aid of gravity should increase nectar-uptake rates at pendulous (downward facing) flowers, yet empirical work in recent years has failed to demonstrate any consistent correlation between nectar extraction rates and flower position (26,29).…”

The hummingbird's tongue: a self-assembling capillary syphon

“…Nectar reservoirs are often shallow, relative to the tongue's groove length, thus precluding tongue submergence, in which case capillary suction is predominantly used. Moreover, the dependence of nectar uptake rates on nectar concentrations reported in the biological literature [8,15,29,41,42] for the hummingbird are well rationalized by the capillary suction model [18]. Guided by our observations, we have developed a theoretical model for the elastocapillary suction of nectar.…”

“…Previous laboratory experiments on hummingbird drinking [8,15,29,41,42] lend support to the capillary suction model [16]. These indicate that the observed dependence of nectar intake rates on nectar concentrations are satisfactorily rationalized by the capillary suction model [18].…”