http://www.woodlands.co.uk/owning-a-wood/managing-your-woodland-for-wildlife/07-chapter-5---creating-new-opportunities-for-wildlife.pdf
5.2 Bird nestboxes
Bird nestboxes can make an enormous difference to the populations of some species, but in many woods they are simply not necessary. First of all you need to assess the availability of natural nest holes in your wood. Mature woodland which has good structural diversity and trees of a wide age range, including older trees with holes and cavities and similar nest sites, is unlikely to benefit from nestboxes. This conclusion may be confirmed if you find evidence of the presence of good populations of a range of hole-nesting birds: consider carrying out a woodland bird survey in spring (see Section 2.3).
If there are already enough nesting places, boxes can actually upset the balance of species if they encourage the populations of commoner birds such as blue tit, which can compete aggressively with much rarer species such as marsh tit for nest sites and food (Symes and Currie, 2005).
Nestboxes might be considered if a particular species such as pied flycatcher or willow tit is declining or absent, in which case boxes could be put up specifically for the target species, especially if suitable nest holes are in limited supply.
Similar issues arise if you have a new woodland at the stage of canopy closure. Here there will be few if any natural hole-nesting opportunities, but putting up nestboxes will attract relatively common species such as blue tit and great tit. These birds normally feed in the higher canopy, but in young woodland, they might compete for food with declining understorey specialists such as nightingale, garden warbler and willow warbler.
In contrast, nestboxes may be much more successful in secondary lowland woodland with poor structural diversity, lacking in older trees, and hence with few opportunities for holenesting birds. Mature woods which have been heavily managed, with few old trees remaining, also lack natural tree holes; examples include lowland mixed deciduous woodland converted to conifer plantations or chestnut coppice with few oak standards remaining; and many of the heavily managed upland oak woods in western Britain. Pied flycatcher and common redstart in particular, which have declined in western oak woods, have benefitted enormously from major nestbox schemes, providing the woodland has sufficient food resources available.
Nestboxes can also be successfully employed for owls in young woodland, or larger open areas associated with mature woodland or plantations. Barn owl boxes erected 3–5 m above the ground on a solitary mature tree or pole may also attract other species such as kestrel and stock dove.