Wild bird nest box size guide
A bird box with a smaller hole is ideal for blue tits.
Similar to blue tits, but with a slightly larger hole in the nestbox.
House martins are famed for living under the eaves of houses, so ideally you would want a nest cup.
Articial nestbox under the eave
House sparrows typically use nestboxes with 32mm holes, but since they are a communal species can also use a colony nestbox.
Nuthatches use a nestbox with a smaller hole.
Robins typically require an open-fronted nestbox that should be hidden behind vegetation.
As starlings are slightly larger birds, the nestbox hole would ideally be approximately 45mm.