2531 INTRODUCTION Earlier literature on bird pollination systems emphasized a dichotomy in nectar properties between hummingbird (Apodiformes: Trochilidae) and passerine (Passeriformes) systems (Cruden and Toledo, 1977; Baker and Baker, 1983). More specifically, hummingbird-pollinated plants were shown to have sucrose-rich nectar, while passerine-pollinated plants were found to have hexose- rich nectars (Baker and Baker, 1983). Initial research into sugar preferences of these two groups of birds found that hummingbirds preferred sucrose solutions (Stiles, 1976; Hainsworth and Wolf, 1976; Martínez del Rio, 1990; Martínez del Rio et al., 1992) and passerines preferred hexose solutions, and indeed in several cases passerines were shown to be sucrose intolerant (Martínez del Rio et al., 1988; Martínez del Rio et al., 1992; Martínez del Rio and Stevens, 1989; Martínez del Rio, 1990; Brugger and Nelms, 1991; Brugger et al., 1993). However, recent research has shown that not only are nectar properties in flowers pollinated by specialized passerines strongly convergent with those of hummingbird-pollinated flowers (Johnson and Nicolson, 2008) but that they too show preferences for sucrose- rich solutions (Downs and Perrin, 1996; Lotz and Nicolson, 1996; Jackson et al., 1998a; Jackson et al., 1998b). By contrast, plants pollinated by generalist avian nectarivores [also referred to as ‘occasional nectarivores’ because they often utilize nectar only as a secondary food source (cf. Johnson et al., 2006)] tend to produce a higher volume of nectar, with a lower sugar concentration and a lower proportion of sucrose, than do plants pollinated by specialist avian nectarivores (Johnson and Nicolson, 2008; Brown et al., 2009; Symes et al., 2009). Specifically, Johnson and Nicolson found that plants pollinated by specialist nectarivores are characterized by low (10–30 l) volumes of relatively concentrated (15–25% w/w) sucrose-rich (40–60% of total sugar) nectars while plants pollinated by occasional bird pollinators are characterized by large volumes (40–100 l) of very dilute (8–12%) nectar, with low (0–5%) sucrose content (Johnson and Nicolson, 2008). In the Americas, this distinction fits the classic hummingbird–passerine dichotomy (Cruden and Toledo, 1977). However, in Africa, where hummingbirds do not occur, this dichotomy applies to passerine- pollinated plants, raising the interesting possibility that generalized and specialized passerine nectarivores differ in their nectar preferences. It has long been suggested that pollinator preference drives selection on nectar rewards (Wykes, 1952; Martínez del Rio et al., 1992), and hence preference experiments have been conducted quite extensively on specialist nectarivorous birds (Downs and Perrin, 1996; Lotz and Nicolson, 1996; Roberts, 1996; Downs, 1997a; Jackson et al., 1998a; Jackson et al., 1998b; Schondube and Martínez del Rio, 2003; Fleming et al., 2004; Lotz and Schondube, 2006). These studies indicate that specialized passerines such as sunbirds have sugar preferences and digestive capacities that are similar to those of hummingbirds (Fleming et al., 2004; Lotz and Schondube, 2006). Most specialized avian nectarivores exhibit either a preference for sucrose, or no preference at high concentrations, no preference at intermediate concentrations, and switch to a hexose preference at low concentrations (Fleming et al., 2004; Fleming et al., 2008; Brown et al., 2010a). However, these switches to hexose The Journal of Experimental Biology 213, 2531-2535 © 2010. Published by The Company of Biologists Ltd doi:10.1242/jeb.040329 Sugar preferences and digestive efficiency of the village weaver: a generalist avian pollinator of African plants T. C. Odendaal, M. Brown*, C. T. Downs and S. D. Johnson School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa *Author for correspondence (brownma@ukzn.ac.za) Accepted 6 April 2010 SUMMARY Recent research has shown that nectar properties of flowers pollinated by generalist avian nectarivores differ markedly from those of flowers pollinated by specialist avian nectarivores. In particular, flowers pollinated by generalist avian nectarivores tend to have very dilute nectar dominated by hexose sugars. To establish whether pollinator-mediated selection can explain these traits, we tested nectar sugar preferences and digestive capabilities of the village weaver (Ploceus cucullatus), a common generalist passerine nectarivore in South Africa. When offered pairwise choices of equicaloric hexose and sucrose solutions, village weavers preferred hexose solutions at 5% and 10% sucrose equivalents (SE) but did not show significant preference for either type of sugar when higher concentrations were offered (15%, 20% and 25% SE). Birds were less efficient at absorbing sucrose than hexose sugars, as revealed by high-performance liquid chromatography (HPLC) analysis of excreta sugar content. This was true at both concentrations tested (8.22% and 25%), although apparent sucrose assimilation rates were still relatively high (89.6±2.9% at low concentrations and 93.6±1.7% at high concentrations). Transit times indicated that sucrose also passes through the digestive tract faster than hexose sugars, particularly when consumed at high concentrations. This may limit the rate at which sucrose can be hydrolyzed before absorption. These results indicate that hexose preferences in generalist avian nectarivores may help explain the low sucrose content in flowers pollinated by these birds. Moreover, the preference for hexose sugars in weavers was most evident at the low concentrations (ca. 9% sugar by mass) that are typical of nectar in flowers pollinated by generalist avian nectarivores. Key words: apparent absorption efficiency, sucrose, hexose, transit time, Ploceus cucullatus. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY