The production of dilute nectars by hummingbird and honeyeater flowers

A survey of data from tropical and temperate regions confirms that nectars of hummingbird and honeyeater flowers are dilute, especially relative to nectars of bee flowers. We use these data, along with theoretical considerations, to examine three recently proposed hypotheses to explain low concentration of hummingbird nectars. None of the quantitative or qualitative predictions of these three hypotheses appears to be upheld. We discuss possible weaknesses of each hypothesis and then present a general framework which may be useful in generating new hypotheses to explain the evolution of nectar concentration. OVER THE PAST FEW YEARS, several authors have discussed sugar concentration of nectars from flowers attracting different kinds of pollinators (e.g., Percival 1965, 1974; Watt et al. 1974; Baker 1975, 1977; Hainsworth and Wolf 1976). One interesting observation is that nectars of hummingbird and honeyeater flowers are dilute, especially relative to nectars of bee flowers (e.g., Hainsworth and Wolf 1972b, 1976; Hainsworth 1973; Stiles 1975; Baker 1975; Percival 1965; Pyke 1979b). Nectar concentrations of 202 plant species known or presumed on morphological grounds to be largely hummingbird-pollinated average 25.4 percent sucrose-equivalent sugars weight by total volume (equals about 23.2% weight by total weight; table 1). Concentrations of 47 honeyeaterpollinated species are similar and average 23.4 percent wt/vol (equals about 21.6% wt/wt; table 2). In contrast, concentrations of 156 largely bee-pollinated species average 41.6 percent (equals about 36.0% wt/wt; table 3). For conversion from percent wt/vol to percent wt/wt see Hodgman et al. (1956: 1925) and Bolten et al. (1979). The most detailed discussions of dilute hummingbird nectars are those of Baker (1975), Calder (1979), and Bolten and Feinsinger (1978). Baker (1975) suggests (p. 39) that "the high energy needs ... [of hummingbirds] will select for richness in energy-giving sugars in the nectars that they favor." He adds (p. 39) that "Increase in sugar concentration necessarily means an increase in the viscosity of the nectar, which could be disadvantageous to a bird that must hover in front of flowers while drinking . (emphasis ours), and that as a result (p. 40) ". tPresent address: The Australian Museum, Sydney, N.S.W. 2000, Australia. 2Present address: Department of Biology, University of California, Riverside, California 92521, U.S.A. a compromise between higher sugar concentration (with smaller volume) and lower concentration (requiring a larger volume for the same caloric content) will be reached in the nectars of hummingbird flowers." Baker also points out (p. 39) that viscosity of a sugar solution increases rapidly above concentrations of about 20 percent (the relationship is exponential, cf. Hodgman et al. 1956: 2042), and implies, we believe, that for these reasons (p. 40) the ". . compromise appears to be struck very often at about 20 to 25 percent [wt/wt; equals about 22 to 28% wt/voll sucrose equivalent." Hence Baker appears to suggest that sugar concentrations of hummingbird flowers have evolved to maximize net rates of energy intake by foraging birds. Baker (1975: 40) also suggests that "Another factor which may limit the strength of hummingbird nectars is the need that these birds have for free water as well as energy-giving sugar for life support." This second hypothesis is pursued by Calder (1979), who suggests (p. 185) that ". . . flowers which have coevolved with pollination by hummingbirds could provide both energy and water balance simultaneously if they produced nectars of appropriate concentrations which depend on environmental temperature," and further (p. 186) that "Since the temperatures to which hummingbirds are exposed range from cold to hot, and the water balance problems range from excess to scarcity, one might look for . . . an in'verse relationship betweenz temperature and optimal nectar concentrationi" (emphasis ours). Calder presents data which (p. 192) ". . . tend to suggest a trend in which concentration may be inversely related to temperature, but the ranges overlap considerably." Bolten and Feinsinger ( 1978) propose a third hypothesis. They suggest (p. 307) that ". . . low sugar concentrations in many hummingbird-pollin260 BIOTROPICA 13(4): 260-27