Bat Box Use and Preference

On completion of the assessment of 239 bat boxes erected as mitigation along the alignment of the AWPR a brief data review was undertaken to see if there were any patterns in use or preference of boxes by bats.

What bat box is best?

A number of species are known to exist in the local landscape including common and soprano pipistrelle, Daubenton’s, natterer’s and brown long-eared bat. Bat boxes included Schwegler Types 1FF (83), 1FW (3) and 2FDP (130) with the number of each type erected presented in parentheses. Some 2FDP boxes were attached to pulley systems allowing them to be erected in tree canopies and hang freely - these boxes were referred to as “high-rise” boxes (23). Using qualified tree climbers, one who held a bat licence, boxes were inspected in May and September.

Across the two visits a number of bats were found occupying bat boxes, whilst the presence of bat droppings also indicated that boxes had been used prior to the check in May and in the intervening period before the September check.

Of the 62 roosts identified in spring, only 8 bat boxes contained bats. All bats existed as individuals and comprised the following sex and species: 6 male soprano pipistrelles, 1 female common pipistrelle and 1 pipistrelle of unknown sex. Of the 38 roosts confirmed in autumn 2015, 13 boxes were found to contain bats of which 5 boxes contained single male soprano pipistrelles while 8 contained more than one bat - in all cases bats were soprano pipistrelles. Sex ratios (male to female) in boxes with more than one bat was as follows: 1:1 (2 boxes); 1:2 (1 box); 1:3 (two boxes); 1:4 (1 box); 1:6 (1 box) and 2:1 (one box). In total 36 bats were identified.

Bat box preference

Accounting for some bat boxes which could not be assessed or found, occupancy of bat boxes is as follows:

Bat box typeNumber of spring roostsSpring Occupancy (%)Number of autumn roostsAutumn Occupancy (%)
1FF0000
1FW0000
2FDP5257%3134%
High Rise1067%747%

Roost discoveries

Although it was expected that the spring check would lead to the identification of more roosts due to the accumulation of droppings since the bat boxes were installed the following pattern of use was noted:

  • 30 roosts identified in spring were still used in autumn
  • 1 bat box not found in spring was subsequently identified as a roost in autumn
  • 7 boxes with no signs of use in spring were confirmed as roosts in autumn

While it would appear that there has been a reduction in the number of roosts between the two periods, the accumulation of droppings since bat boxes were erected would account for the greater number of roosts found in spring. This trend may be further exacerbated by bats investigating numerous boxes when they were first erected before only occupying those which were preferable.

Roost preference also changed during the year with seven boxes not used in spring inhabited in autumn. The change in use is also supported by the number of boxes containing bats and the numbers of bats in each box across surveys. Individual bats were recorded in spring but roosts comprising up to 7 bats were found in autumn with a total of 36 bats caught. This observation corresponds to the formation of mating colonies in autumn, further supported by boxes containing one male bat and several females.

Based on bat signs in 2015 and the proportions of bat box available it would appear that there is a preference for high rise followed by 2FDP bat boxes. No bats were found in the 1FF or 1FW bat boxes, although bird droppings and roosting blue tits were found in several of the 1FF boxes.

Future mitigation design

Whilst the information presented does have its limitations, principally being based on only one year of data, it does indicate some interesting patterns of bat box use and preference. A more rigorous assessment could be completed if data from previous assessments can be sourced. Whilst high rise bat boxes may be of particular value to pipistrelle bats, other types may play a significant role immediately after their erection, several years later, or once discovered by species existing at lower population densities and should therefore not be discounted.

It is important that any large scale bat mitigation should provide sufficient diversity in the type of bat box used, as well as address the more familiar characteristics associated with geography and aspect. It would also be useful for mitigation to be designed which allows for scientific testing so that it may be possible to get meaningful results and inform future mitigation strategies. This may not only ensure the longevity of local bat populations but also demonstrate to any Client how existing information is being used to ensure the cost efficiency of mitigation they may be obliged to undertake.

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