Tree hollow
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. Hollows in the trees also appear under the influence of water, fire, wind, lightnings and living beings, such as rodents, mammals (it can take centuries to form the hollow fit enough for the big mammal), birds, some bacteria and fungis.
Tree Hollow Media
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.