The feet of perching birds can perch even on weak, slippery surfaces due to the rough bumpy skin on their soles.

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"The Passeriformes or 'perching birds' have the typical bird foot: three toes forward and one behind, with which a bird can perch crosswise on a branch. A bird's sole is covered with rough bumpy skin, so that it can obtain purchase even on a small, weak, mobile twig which may be wet and slippery after rain." (Foy and Oxford Scientific Films 1982:183)

Book
The grand design: Form and colour in animalsBLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, LondonAugust 18, 1983
Sally Foy

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Living System/s