Introduction

Naturally bats roost in a wide variety of cracks and crevices from old woodpecker holes in trees to caves and fishers in rocky outcrops. Some species have adapted well to man’s impact on the landscape and make use of buildings, bridges and mines, but for many species the loss of woodlands, and in particular hollow trees, has had a serious detrimental impact on bat populations. Bat boxes aim to provide suitable roosting sites for bats throughout the year.

 

Types

In the UK bats hibernate from around October/November through to April/May. Consequently their requirements for roosting sites change dramatically depending upon the time of year. The main difference between bat boxes built for summer use and those built for winter use is the wall thickness. Winter (hibernation) boxes require a wall thickness of at least 10 cm to provide sufficient insulation. In practise, hibernation boxes are only rarely used.

 

Summer boxes tend to have a wall thickness of about 2.5 cm. The designs and dimensions vary but there are a few important criteria. Firstly, these boxes need to be large enough to accommodate large clusters of bats. This is particularly important for nurseries where young bats cluster to conserve heat. The box must have a well fitting lid and good joints - bats don’t like drafts! The entrance slot should be large enough to allow the bats to enter without being so large that birds use the box for nesting and roosting. A slot 15 mm wide and the width of the box is usually adequate. Finally, the box must be correctly positioned with no branches impeding the entrance and high enough for the bats to find. This will be discussed in more detail below.

 

Construction

The bat box below (modified from Stebbings and Walsh, 1991) is a very widely used and successful design for general species use. Other designs may be more suited to particular species. With the appropriate tools, it should take less than an hour to construct.

 

Select a soft wood plank 1,200 mm x 150 mm x 25 mm and mark out as shown. Red woods are particularly durable, rot-free and light weight but can be expensive. The timber used to make the box below was tanalised with an environmentally friendly preservative which is believed to be safe for bats. It cost less than £4 for a plank 2400 mm long (sufficient for making 2 boxes).

 

 

 

If wood is smooth cut thin grooves 10 mm apart and 1 mm deep in the back as shown.

 

 

Also cut a 28 mm wide grove 3 mm deep, starting 50 mm from the top. This is for the lid to fit into. The finished back should look like this.