Sugar Preferences of a Nectarivorus Passerine Bird, the Lesser Double- Collared Sunbird (Nectarinia Chalybea)

Abstract

1. In contrast to hummingbirds, passerine frugivores prefer hexose sugars to sucrose and absorb sucrose poorly. It has been predicted that the sugar preferences and digestive physiology of specialized passerine nectarivores will be similar to those of frugivores. 2. We examined the preferences of 11 Lesser Double-collared Sunbirds, Nectarinia chalybea (Nectariniidae), for 20% (w/w) solutions of sucrose, glucose, fructose, a 1: 1 glucose+fructose mixture, and xylose. The order of preference was found to be sucrose = glucose + fructose = fructose > glucose > xylose. The birds rejected xylose, which has recently been found in the nectar of Protea and Faurea (Proteaceae). 3. Preferences for different sugar concentrations were also tested. In the case of sucrose and fructose, the sunbirds preferred 20% to 10% solutions, but were indifferent to 20% vs 30%. They preferred 20% to 30% glucose, but were indifferent to 10% vs 20%. 4. The concentration and sugar composition of the cloacal fluid were measured after feeding on known diets. The sunbirds absorbed sucrose, glucose and fructose from the ingested food at close to 100% efficiency, but xylose was excreted. 5. The results of this study thus contradict the belief that passerine nectarivores are inefficient at digesting sucrose and reject sucrose in favour of hexose sugars.