Whilst trees can be of benefit by mere virtue of their qualities, which in the case of owls may be with the presence of good-sized cavities and hollows, there are times when man can artificially add to the suitability of a tree in terms of its habitat value. For owls, this can principally be achieved through two means: (1) undertaking veteranisation techniques to create necessary cavities or hollows within a tree, or more routinely (2) attach owl boxes to the main stem or lower crown of a tree.
Below, we can see an example of the latter. In fact, this owl box was put up just two years ago (from memory), and last summer there was a young female barn owl occupying the box, where her first ever set of chicks (a total of three) were successfully raised (all survived). We know this because, when she was young, she had herself been tagged, and the nest box was surveyed, during the summer, by the RSPB. The oak the box is attached to is however rather exposed, though the nest box faces north and, in spite of the constant foot traffic beneath, the female barn owl found the box acceptable for a nesting site. This may perhaps suggest one of two things: (1) this site is perfectly acceptable, or (2) there are no more desirable sites available, which led to this female using this site for nesting. Regardless, this box has had its first success!
Below, I have attached a few photos for those interested.
Arboriculture 