Trees develop hollows for a number of reasons, some being, decay, wounding, termites, ants etc. The discovery of a hollow, does not automatically signal the need for whole tree removal.
Arboriculturists use a range of assessments criteria and additional diagnostic tools that can be applied to determine a tree’s safety status. Firstly the species of tree is noted; the size and extent of the hollow in relation to stem size, diameter and height and what targets are within range in the event of failure. If this preliminary assessment does not adequately quantify safety margin, he or she will undertake diagnostic investigation. Trees which are not deemed to pose serious threat to persons or property are usually retained. Hollows in trees are very likely to provide habitat for a wide range of vertebrates, invertebrates and other organisms.
Article thanks to Margot Blues