A naturally formed tree hollow at the base of the tree
A tree hollow or tree hole is a cavity in a tree. Such hollows naturally form in the branches or trunks of many kinds of trees. They provide a habitat to animals, such as owls.
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Multiple tree hollows in a plane tree in Baden, Austria
Indian spotted owlet in tree hollow
A river red gum, with hollows. The younger trees surrounding it would generally not yet have developed hollows suitable for vertebrate species.
Hollows excavated by the lesser spotted woodpecker (Picoides minor). Also fungus, which may also help in the formation of hollows through the decomposition of heartwood.