Size of entrance
An entrance should let Swifts in and keep other species out, particularly Starlings. If an entrance with width equal to depth, e.g. circular or square, allows Swifts to enter, then Starlings can probably enter too. Conversely any such entrance small enough to keep Starlings out, may make life difficult for Swifts.
Some commercial boxes, such as the Schwegler 1MF double box and the Schwegler 16 have 45mm entrances that Starlings can easily enter.
Even our hero, Edward Mayer, recommends a home made box with a 45mm circular entrance, which Starlings will love. If you like Starlings, and we do, then put up boxes for them, but well away from any nesting Swifts, as Starlings can get aggressive during the brief period before their young have fledged and after the first breeding Swifts have arrived.
|Shropshire Swift Box|
The critical dimension is the 30mm depth, the width does not matter. Some boxes have an entrance across the whole width of the box, e.g. the Shropshire Swift Box
A larger entrance, with a Starling-proof tunnel inside 100m long x 100mm wide by 33mm high, is sometimes recommended. Starlings with long legs cannot crawl through the tunnel, but Swifts with very short legs can. However, it seems unnecessarily complicated when a simpler solution exists, but it is a way forwards if a box has been built with too large an entrance. However, there are usually simpler ways of reducing the entrance size to keep Starlings out.
Direction of entrance
Another consideration is the direction the entrance faces. Although a horizontally facing entrance will not prevent Great Tits or House Sparrows from using a Swift nest-box, an entrance facing vertically downwards makes life more difficult for them. Although we have heard of many instances of Great Tits nesting in Swift boxes with horizontal entrances, we have rarely experienced one in an entrance facing down. Although Swifts can enter an entrance facing downwards, it is thought helpful to provide an adjacent landing platform below the entrance against which the Swifts can brace themselves.
Shape of entrance
All entrance shapes seem to be acceptable, and this is more dictated by what is easiest to make. The entrances in the Oxford Museum Tower are either 45mm square, or a D-shape 45mm x 45mm. An analysis of occupancy rates shows that the Swifts clearly prefer the square entrances, probably because they are slightly larger in area, and therefore easier to enter. Of course, such entrances will also allow Starlings in, even though they face vertically downwards.
|Canopy to provide shelter and protection from above|
When Starlings attack Swifts returning to their boxes, the attack normally comes from above as the Starling tries to force the Swift to the ground. Some defence can be provided if the entrance has a canopy above it. Downwards facing entrances, and boxes under broad eves may also help this.
|Zeist Swift Box with entrance facing 45° down|
The Zeist box has an entrance facing 45° downwards, with an overhanging lid and Swifts readily take to it. We made similar entrances for our project at Milton Road Primary School.
Distance to floor of nest box
The question arises as to how far the entrance should be above the base of the box. Entrances in the floor of the box provide no difficulty getting in or out, but nestling Swifts may find it more easy to fall out accidentally, and this has happened at least once with my own boxes with entrances in the floor. I have considered putting a small "door step" inside the entrance. Entrances high in the wall of the box should only be entertained if the inside wall is made of a rough soft material, such as very rough timber, for the Swifts to get their claws into - otherwise the nest-box may become a death trap - this has been known to happen.