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

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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 nectar-feeding 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. Dark-capped 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.

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Acknowledgments

Tamsanqa Majwara is thanked for his assistance in animal capture and care. Birds were captured under permit from Ezemvelo KZN Wildlife. The project was approved by the animal ethics committee of the University of KwaZulu-Natal.

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Authors and Affiliations

  1. School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3201, South Africa

    Mark Brown, Colleen T. Downs & Steven D. Johnson

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  1. Mark Brown
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  2. Colleen T. Downs
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  3. Steven D. Johnson
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Correspondence to Mark Brown.

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Communicated by C. G. Guglielmo.

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Brown, M., Downs, C.T. & Johnson, S.D. Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor . J Ornithol 151, 637–643 (2010). https://doi.org/10.1007/s10336-010-0498-8

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  • DOI: https://doi.org/10.1007/s10336-010-0498-8

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