Food Value and tree selection by Glossy Black-Cockatoos Calyptorhynchus lathami

Authors

  • Gabriel M. Crowley,

    Corresponding author
    1. Department of Environment and Heritage, PO Box 39, Kingscote 5223, South Australia and
    2. Department of Geography, University of Adelaide, Adelaide, South Australia, Australia
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  • Stephen T. Garnett

    Corresponding author
    1. Department of Environment and Heritage, PO Box 39, Kingscote 5223, South Australia and
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*Corresponding author. †Present address: Queensland Parks and Wildlife Service, PO Box 2066, Cairns, Qld 4870, Australia. (Email: garnettcrowley@ledanet.com.au)

Abstract

Glossy Black-Cockatoos (Calyptorhynchus lathami) appear to maximize dry matter intake by selecting feeding trees that have the most profitable seed crops as indicated by Clout’s Index (seed weight/total cone weight). However, as it is unlikely that cockatoos can directly assess Clout’s Index, the mechanism for such selection is unclear. Moreover, as cockatoos consume only the kernels, and not all seeds contain kernels, better estimates of food value are required. Therefore, we examine seed and cone characteristics and establish that Seed Fill (percentage of seeds containing kernels) and Kernel Ratio (average kernel weight/average cone weight) contribute significantly to Food Value (weight of kernels/total cone weight). We propose that these factors can be rapidly assessed by cockatoos, and show that selection of feeding trees can be more accurately predicted using discriminant analysis with a combination of Seed Fill and Kernel Ratio than with either Food Value or Clout’s Index alone. Along with most other characteristics, Seed Fill and Kernel Ratio were consistent between the two years of study, enabling foraging cockatoos to return to profitable trees annually, without sampling. Where sampling is undertaken, rapid assessment of profitability by sampling cone ends is possible, as kernels are randomly distributed through the cone. Also, a decline in Food Value, Seed Fill and Kernel Ratio with cone age, means that cockatoos could also assess profitability on the basis of cone colour. We show that concentrations of individual nutrients are unlikely to contribute to tree selection, previous reports of such selection being caused by the predominance of protein and oils in the kernels, and of ash, fibre and carbohydrates in the samara. We therefore conclude that cockatoos select feeding trees primarily on the basis of optimizing kernel intake.

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