How big is a Swift nest-box?

Update 28th July 2012
We have usually assumed that Swifts prefer larger spaces to smaller spaces. However, some recent experiences seem to indicate that smaller boxes may be OK. 
Our installation of air brick liners in the old factory site in Brook Street St Neots gave us 12 nest places with a floor area of 200 x 175mm and a height of 100mm. At least two pairs of Swifts took up residence, and one may have produced a chick.

We recently examined the contents of the nest-boxes in St Mary's Ely. We compared two basic sizes of box: 

• 16 narrow boxes are  100mm x 300mm (a few widened to about 125mm) 
• 80  wide boxes are 200mm x 400mm (most of these only recently installed).

In the narrow boxes we found 9 boxes containing 15 chicks = 1.67 chicks per box

In the wide boxes we found  9 boxes with 14 chicks = 1.75 chicks per box

The narrow boxes have been installed for a longer period than the wide ones which is why occupancy rates are different. Now, we do not know how many other boxes had failed; another uncertainly is that the narrow boxes have entrances in the floor, whereas the wide boxes have entrances in the side, but it seems that swifts are as productive in narrow boxes as wide ones.

Of course, we need to collect more data, but perhaps we need to be a little more open minded about small and narrow boxes.

Posted by Dick Newell:

Swifts can, apparently, nest in anything from a House Martin's nest to a whole roof space though the success rate in the former may be questionable. They nest in holes in trees in Scotland and in northern Europe in what must be quite confined spaces. Examples of successful commercial nest boxes have internal dimensions like 140mm x 300mm floor area, height 140mm. Erich Kaiser's very successful colony in Kronberg, Germany, has large nest-boxes about half the size of a tea-chest.

There are some very small commercial Swift nest-boxes, with a width of about 100mm, but there does not seem to be any evidence of their effectiveness; Swifts can turn in a space this small, but with extreme bending of feathers (per Deborah Lauterpacht). Swifts have been seen nesting in small ventilation pipes down to 75mm (3 inches) in diameter. They cannot turn in a space this small, so they reverse out (per Mark Smyth). When Swifts nest under tiles in a roof, they have very little headroom, but they are successful. When Swifts get into a large roof space they sometimes nest several feet from the entrance, even though there is space nearer the entrance. Swifts can sometimes access their nest place through a conduit several feet long.

Coming at the question from a different direction, what is a Swift likely to need? A Swift's wing length (the distance between the carpal joint and the wing tip) is ~175mm (7 inches). So, If a Swift is to turn around without bending its wing feathers then it requires a space no narrower than 175mm, though they can avoid bending their feathers in a narrower box by raising the wing tips up the wall. Fledgling Swifts build up the strength of their flight muscles by spreading their wings and doing push ups. The span of their spread wings while doing this could be ~250mm (10 inches) ~330mm (13 inches).

On occasion, intruders can get into an occupied box, when an almighty fight may ensue (see here). If there is insufficient space to manoeuver, these fights can end in deadlock. Towards the end of the breeding season, there may be 3 full grown fledglings and two adults roosting in the box.

Where space is restricted, then the minimum width below which one should not go may be 100mm (4 inches), but this needs to be tested before one can recommend it with confidence. So, where space is available, then the minimum internal dimensions should be something like 175mm x 300mm (7 inches x 12 inches). The diagonal of such a box is 347mm, which is sufficient for an exercising Swift. If more space is available, the Swifts would prefer it.

One last consideration is that if one wishes to incorporate a camera in a Swift box, a larger or longer box may be preferred to obtain a good view of the whole space that the birds occupy from the entrance to the nest, though beware excessive glare if a camera is pointed at the entrance.