Air and bird trees

A close up picture of a tree beginning to grow on the branch of another tree

When birds eat holly or rowan berries and then roost on a mature oak limb the seeds inside the berries will eventually pass through the bird and can land on an area of branch or hollow of the trunk where soil has accumulated.

The seed can then germinate and a small seedling can grow forming a bird tree.
Air trees are very similar but not produced by birds but by the wind blowing tiny seeds from birch or other small seeded trees onto suitable locations in mature trees.
In both cases these trees do no harm to their host. If they are lucky, and the host tree has an exposed decaying trunk, they will send roots into this decaying wood which can eventually reach the ground.