Concentration and temperature effects on sugar intake and preferences in a sunbird and a hummingbird
Fleming, P.A., Hartman Bakken, B., Lotz, C.N. and Nicolson, S.W. (2004) Concentration and temperature effects on sugar intake and preferences in a sunbird and a hummingbird. Functional Ecology, 18 (2). pp. 223-232.
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1. The nectars of hummingbird- and passerine-pollinated plants differ in both sugar type and concentration. Firstly, hummingbird nectars tend to be more concentrated than passerine nectars. Secondly, sucrose dominates hummingbird nectars, whereas glucose and fructose (hexose sugars) are the prevalent sugars in nectar of almost half the passerine-pollinated plants. It has been commonly assumed that these differences can be attributed to selection pressure from birds, largely determined by avian sugar preferences and digestive physiology.
2. Intake and preferences of Whitebellied Sunbirds Nectarinia talatala (A. Smith) and Broadtailed Hummingbirds Selasphorusplatycercus (Swainson) were examined across a range of sucrose and equicaloric hexose solutions at two ambient temperatures. Hummingbird energy balance was not affected by sugar type. Sunbird energy balance was only influenced by sugar type on a very dilute diet (0.1 mol l-1 sucrose equivalents, 'SE'), when the birds ingested 12% more sucrose than hexoses. Sunbirds and hummingbirds showed similar patterns in sugar preferences. Sunbirds preferred hexoses when offered dilute diets (0.1 mol l-1 SE at 21°C) and showed slight preference for sucrose when offered more concentrated diets (significant at 0.75 mol l-1 SE). Hummingbirds showed slight (non-significant) hexose preference when offered a dilute diet (0.25 mol l-1 SE at 10°C), but, in contrast to previous findings, hummingbirds showed no significant sucrose preference.
3. Our findings for Whitebellied Sunbirds and Broadtailed Hummingbirds do not support an ornithocentric explanation for nectar composition. Plant physiology and opportunist nectar feeders may also be influencing nectar sugars. To further address this question we discuss methodological considerations for researchers investigating how avian preferences and physiology may affect nectar composition.
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