Sugar preferences and 'side bias' in Cape Sugarbirds and Lesser Double-collared Sunbirds

Abstract

Using pairwise feeder tests, we studied preferences for sugars in Cape Sugarbirds (Promerops cafer) and Lesser Double-collared Sunbirds (Nectarinia chalybea). Birds were offered 20% (w:w) solutions of sucrose, fructose, glucose, and a mixture of equal parts of glucose and fructose. Cape Sugarbirds showed no preference among these sugars, whereas the order of preference in Lesser Double-collared Sunbirds was sucrose > fructose = hexose mixture > glucose. Both species showed strong 'side biases,' with individuals consistently drinking more from feeders offered on a particular side of the feeder pair. We suggest that this bias is a manifestation of stereotyped foraging behavior rather than lateralization or true 'handedness.' The absence of a sucrose aversion in 'fynbos' (i.e. Cape Floristic Kingdom of southern Africa) nectarivores such as sugarbirds and sunbirds is not surprising because the fynbos is characterized by high floral diversity and low bird diversity and by the occurrence of both sucrose-dominant and hexose-dominant nectars. However, our findings contradict an earlier generalization that passerines prefer hexoses to sucrose. This generalization is based on studies of several American and European species and of one East African species, and it may be confounded by comparisons of specialized nectarivorous non-passerines with generalized frugivorous/nectarivorous passerines. In separate feeding trials, both sugarbirds and sunbirds showed a strong aversion to the pentose sugar xylose, a nectar sugar newly described for the Proteaceae. The reason for the occurrence of xylose in nectar of the Proteaceae is unknown.