Putting up nest boxes is a great practical and satisfying way that we can help birds which always feels good, especially if you build the boxes yourself and later see them being used. From a conservation point of view, box schemes have been really important for the recovery of populations of certain species, and in some cases, a shortage of nesting sites is the most limiting factor for birds. It’s worth remembering, however, that the great majority of bird species do not nest in cavities, and even for those that do, food supply and habitat have to be good before it’s worth considering providing boxes. Putting up lots of nest boxes for Blue and Great Tits in urban gardens, for example, can result in more birds trying to breed than can be supported by the local environment. Bird feeding through the winter means that plenty of these birds survive and look to breed in the spring, but the lack of native trees typical of these settings do not produce enough caterpillars to ensure success of the nests. The result is that fewer chicks are produced and those that do survive are weaker than chicks reared in natural woodland situations – so do bear in mind whether nest boxes are appropriate to the site before embarking on a scheme. The other thing to watch out for is whether you might be providing a home for a less welcome occupant. In some settings, boxes put up for birds have been taken over by Grey Squirrels, which are notorious for raiding the nests of other birds in the vicinity and not something we want to encourage.
Plenty of studies have shown, however, that natural nest holes are a dwindling resource in Britain. Modern building standards and home renovations often remove the little gaps in roof spaces where birds like House Sparrows, Starlings and Swifts would once have nested; a recent survey of Cambridge showed that House Sparrows were far more abundant in the least affluent parts of the city, despite the fact that the ‘posh’ parts had lots of green spaces and everything else that sparrows required, it was the eaves of the cheap houses that provided nesting opportunities and determined where sparrows were found.
Out in the countryside, the conversion of old barns to desirable human dwellings has taken the aptly-named Barn Owls home away; pioneering work by Colin Shawyer has reversed this situation through the provision of large nest boxes suitable for these birds, and numbers of Barn Owls have doubled as a result. It is thought that boxes now make up a large proportion of the nesting sites used by these birds. Where woods and orchards are over-managed, removing dead trees (sometimes thought to be dangerous or worthless) then a lack of suitable holes can be very restrictive on the types of birds that breed. A particular concern is for those migratory species which require holes to breed – birds like the Pied Flycatcher and Redstart. By the time they arrive back from Africa, all the best holes have been taken by the resident species, particularly Great Tits, and the migrants have nowhere to nest. It has been known for Great Tits to kill Pied Flycatchers that tried desparately to take over an occupied nest site! In the areas where these birds occur, mainly the oak woods of western Britain, nest box schemes have been hugely successful in increasing the numbers of the migrant species.
My advice for National Nest Box week is to enjoy it, build and put up a box or more, but make sure that it is the right sort of box in the right sort of place, and always keep an eye on the results of your efforts. A careful watch of a nestbox from a safe distance will soon tell if it is host to the right species or not, and a check at the end of the breeding season (in autumn) may well tell you if they were successful. The best book for nest box designs and information is the BTO nestbox handbook but there is some information about boxes on the BTO website, http://www.bto.org/about-birds/nnbw/make-a-nest-box, and also box designs are available on the RSPB website https://www.rspb.org.uk/advice/helpingbirds/nestboxes/