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