Birds and Ancient Trees
Veteran trees provide birds with more essential resources than other trees. These include food sources, in the form of invertebrates associated with decaying wood or bark, together with tree cavities, which can serve as nest sites for a wide range of birds.
Some birds – such as owls, kestrels, marsh tit and tree-creeper – adopt existing cavities with little or no modification, while others – including woodpeckers and nuthatch – modify the cavity and its access considerably. Modification of cavities can influence the range of invertebrates living in them.
Some birds, including foliage gleaners such as titmice and warblers, depend directly on trees for the bulk of their food, while others are specialists, which feed on invertebrates associated with decaying wood and on invertebrates which are merely sheltering there (e.g. in winter or at night. Generally the bird does not concern itself with the reason why the particular invertebrate is where it is, merely eating it! Some birds, mainly woodpeckers, break into decaying wood in search of food but most others, – even nuthatch and tree-creeper – usually glean their prey from external surfaces and shallow cavities.