Sunday, 23 August 2020

Effect of nest box width on wing bending

A swift's wing length is about 170mm, thus any box with a minimum dimension less than 170mm will require the swift to bend its wings in order to turn. This post is a brief analysis of just how much a wing needs to bend for a range of box widths.

A number of nest boxes on the market have a minimum dimension of 100mm, which requires a wing to bend over 180° in order for the bird to turn.

The following charts illustrate the relationship between. box width and degree of bending. These are calculated with an idealized assumption that a straight wing would bend into a circular arc. Of course, most of the bending and wear will be concentrated nearer the wing tip which might be worse than it being uniformly distributed along the wing length.

Swifts will nest in boxes 10cm wide, but the wear to their wingtips after a whole season of turning in such boxes is unknown.

Relationship between box width and bend angle
The range of shapes for different box widths. All of these lines are the same length, 170mm.

Thanks are due to Mark Smyth for these videos showing a Swift turning in a 10cm wide nest box:


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Sunday, 2 August 2020

Swifts inside PVC soffits

Open eaves, with breeding Swifts, are frequently turned into closed eaves with PVC soffits. This is an example of such a project in Cambridge in 2020.

The pictures below show 5 neat holes in the Soffit of Gray House, a block of flats in Cambridge. Holes 1, 3 and 5 have an S Brick eaves box above them which contain any birds that enter these holes and they act as a barrier for any birds that might enter holes 2 and 4. There is a total of 8 S Brick eaves boxes in this building with 10 spaces for swifts.

You can read about the S Brick for closed eaves here:

This includes a link to instructions on how to make a neat entrance in a soffit

5 neat holes (click to enlarge)

There is an eaves box above the left hole,
nothing above the right hole