Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor

Abstract

It has recently been recognized that flowers pollinated by generalist opportunistic nectarivores tend to have different nectar properties to those pollinated by specialist nectarivores (including both hummingbirds and specialist passerines). While renewed interest in specialist avian nectarivore sugar preferences and digestive physiology has helped explain the concentrated sucrose-dominated nectar of plants they feed on, there has been little progress in understanding why generalist or occasional nectarfeeding birds tend to be associated with flowers that have dilute hexose-dominated nectar. We examined sugar preferences and assimilation efficiencies over a range of concentrations, and concentration preferences, in Dark-capped Bulbuls Pycnonotus tricolor, one of the more common occasional avian nectarivores in southern Africa. Darkcapped Bulbuls showed significant preference for hexose sugar solutions, irrespective of concentration, when given a choice between hexose and sucrose solutions in equicaloric pair-wise choice tests conducted at five different concentrations (5-25%). This contrasts with results from specialist nectarivore groups which generally show a significant concentration-dependant switch in preference from hexose at low concentrations to sucrose at high concentrations for equicaloric solutions. In addition, Dark-capped Bulbuls showed an unusual lack of preference for solutions of higher sugar concentration when simultaneously offered four solutions varying in concentration from 10 to 25%.Dark-capped bulbuls also showed a unique effect of concentration on sugar assimilation efficiency, assimilating relatively more energy on 5% diets than on 25% diets. Although able to assimilate sucrose effectively, assimilation rates of hexose sugars were marginally higher. These results shed new light on pollination systems involving occasional nectarivores and, in particular, help to explain the prevalence of low concentration hexose-dominated nectars in flowers pollinated by these birds.

Nectar palatability can selectively filter bird and insect visitors to coral tree flowers

“…Dark-capped Bulbuls show even stronger hexose preference, choosing hexose rather than sucrose solutions at any nectar concentration (Brown et al 2010a). In the present study, however, they did not differentiate between sucrose and hexose solutions.…”

Effects of nicotine on the digestive performance of nectar-feeding birds reflect their relative tolerance to this alkaloid

“…Brown et al (2010) reported a much lower AE* of 65% for dark-capped bulbuls fed 0.7 M sucrose, and we are unable to explain the discrepancy. Izhaki (1992) measured AE* values of 78-85% in yellow-vented bulbuls P.…”

“…The apparent assimilation efficiency found for dark-capped bulbuls on a sucrose diet was high and is interesting considering the preference of this species for hexose sugars (Brown et al 2010). Their nicotine tolerance, however, was unaffected by sugar type (sucrose vs.…”

Sugar preferences and digestive efficiency of the village weaver: a generalist avian pollinator of African plants

“…From the latter group, it is evident that generalist avian nectarivores, although they possess some ability to digest sucrose, prefer hexose sugars. However, apart from recent work on bulbuls by Brown et al (Brown et al, 2010b), only comparisons at single concentrations have been reported, and these concentrations have not been representative of the very dilute nectar found in flowers pollinated by these birds. Also, test solutions in earlier studies were not equicaloric; thus, conflating tests of sugar preference with an energy difference between test solutions (Brown et al, 2008).…”

“…These can be divided into those concerned only with members of the Muscicapoidea superfamily, lacking sucrase activity (Schuler, 1983;Martínez del Rio and Stevens, 1989;Brugger, 1992;Malcarney et al, 1994;Gatica et al, 2006), and those concerned with other species Franke et al, 1997;Lane, 1997;Mata and Bosque, 2004;Brown et al, 2010b). From the latter group, it is evident that generalist avian nectarivores, although they possess some ability to digest sucrose, prefer hexose sugars.…”