Tree hollow

A cavity in a living or dead tree, formed by decay, damage, or growth. Tree hollows provide shelter and nesting sites for many animals.

Hollow-nesting bird: A bird that nests in cavities in trees or other structures. Parrots, owls, and many other birds depend on tree cavities for breeding.
Wood decay: The breakdown of wood by fungi and other organisms. Decay removes heartwood and can enlarge cavities within a tree.
Heartwood: The nonliving inner wood of a tree trunk, often darker and less involved in water transport than sapwood. Cavities commonly form as this inner wood decays.
Habitat tree: A tree that provides habitat features such as cavities, loose bark, or dead branches. A hollow-bearing tree is a prominent kind of habitat tree.
Tree squirrel: A squirrel adapted to living and moving in trees. Some tree squirrels use hollows as dens and places to raise young.
Heart rot: Wood decay that affects the central heartwood of a tree. When heart rot advances, a tree may retain a living outer shell around a hollow core.
Sapwood: The younger, outer wood that transports water and minerals in a tree. A hollow can coexist with functioning sapwood surrounding the cavity.
Keystone structure: A physical feature that supports ecological functions or species disproportionate to its abundance. A few hollow-bearing trees can provide scarce nesting sites for many species.
Microbat: A small bat in the traditional group Microchiroptera, many species of which roost in cavities. Tree hollows provide roosts for bats that shelter singly or in colonies.
Tree wound: An injury that breaks or removes a tree's protective outer tissues. Broken branches and other wounds can expose wood to decay organisms.

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