How to identify bird and mammal tree holes

It's easy to spot holes in trees, but do you know who lives there? We explain how to identify which hole belongs to which species.

How to identify wildlife signs

In March, many birds and mammals are digging holes in trees (or ousting their established occupants) as a prelude to breeding.

Don’t approach or disturb any holes you see – use binoculars to look for telltale identification signs. If in doubt, wait and see who returns to the hole.


  • Woodpeckers generally excavate new holes each year, often in wood weakened by fungal attack. Holes are usually a few metres up, but can be quite close to the ground.
  • Some wood chips can be seen at the base of the tree. It can take two weeks to dig out the nesting chamber.
  • Entrance holes are nearly spherical and just big enough for a woodpecker to slip through – 5.5cm diameter for great spotted woodpecker, 6.5cm–7.5cm for green.
  • Cavities from previous years are used by a wide range of other species; starlings can oust woodpeckers from their holes.
  • Treecreepers live in large trees. They make small nests concealed behind loose bark or in crevices.
  • The nest is a loose cup, 7.5cm across, made of twigs, roots, moss and grass.
  • Bats such as noctules and serotines roost in tree holes, often in old woodpecker holes or natural crevices.
  • The trunk below the hole is darkly stained with droppings and urine.
  • In tight crevices, the sides of the hole are often greasy where the bats squeeze in.
  • Nuthatches use natural crevices or woodpecker nests. The mouth of the nest hole is reduced in size with mud mixed with saliva (very hard when dry) to leave a hole about 3cm across.
  • Nuthatches block up holes in nestboxes in the same way.
Grey squirrel
  • Grey squirrels mainly use dreys, but will gnaw small cavities in trees, often in rotten wood where a dead branch has fallen off.
  • They like dense ivy cover. Teeth marks are generally visible around the edge of the hole.
  • Starlings use holes at any height in trees, cliffs and walls. In trees, they often use woodpecker holes.
  • The untidy nest of straw, dead grass, feathers etc is often visible. You may also see droppings on the tree trunk (under the hole) squirted out by nestlings.
Tawny owl
  • Tawny owls generally nest in tree cavities, but will use rock crevices and nests of other birds.
  • The holes are often much larger than the owl.
  • Tits generally use natural holes but some, such as willow tits, may dig their own in rotten wood.
  • Blue and great tits use holes in trees or walls from ground level upwards. The hole is often only a crack through which the bird squeezes, with no obvious sign other than slightly worn bark around the hole.


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