BAT BOXES
HANGING INSULATED BAT BOXES.
Back in 1994 I set up 15 conventional wooden bat boxes, hung three to a tree, in a valley-bottom, mixed deciduous woodland that Cornwall Wildlife Trust owns in East Cornwall. I’d no clear idea of the purpose other than to see what bats might be there. It was good old woodland so I didn’t expect to get many bats in the boxes as I thought they would prefer the opportunities offered by the habitat. (In those days we generally only had heterodyne detectors, so acoustic surveying wasn’t as sophisticated as it is now.) But then I advertised the annual October inspection among my friends in a natural history group I was involved with, as well as our Cornwall Bat Group. So I was able to demonstrate wild bats in the hand. This morphed into training sessions for prospective volunteer bat roost visitors. And in 2010 I added another annual inspection in May.
A bottom-opening, cubic wooden box with Brown Long-eared Bats.
Take-up was low: I was doing well to have bats in more than one box. I had more Blue Tit nesting remains and Dormouse nests than bats! And if I did get bats the numbers were always low, generally singles. But in later years, 2010 on, numbers increased and I started getting up to 15 Natterer’s (Myotis nattereri) or Brown Long-eared (Plecotus auritus). (As the cubic boxes aged they were gradually replaced, with bottom-opening or wedge-shaped designs.)
But in 2007 I made some open-bottomed beehive-type boxes from scrap insulation panels of various types, cased in ply, and suspended from tree branches. The idea was to see if bats were interested in better insulation (have you ever sat on a piece of Styrofoam?), and to make the bats visible from the ground without having to open the boxes, and thus requiring a licence to survey. I figured that if they worked it would be a more efficient way of censusing the bats in the area, because one could visit at any time (I didn’t fancy opening boxes during the breeding season or during hibernation, so I was restricted to spring and autumn.) So I tried four boxes of various dimensions for a start.
Take-up was immediate. At the first visit after hanging two of the four had single bats, a Noctule (Nyctalus noctula) and a Pipistrelle. 50% occupation at the first inspection! That compares with about 6% for the wooden cubic boxes. (You can’t handle the bats and they are not always easy to see from 10-15 ft. away, so identification is somewhat tentative.) Subsequently, occupation has been running at between 50% and 75%.
Insulated, beehive-type, hanging boxes. They are not pretty, but I did not want to spend any money or too much time on them until I found out if they worked! Old and replacement beehive boxes.
Inspecting a box from the ground.
But now I don’t have to depend on help carrying a ladder, so I visit the site more often, winter and summer included. And occupancy is generally low during hibernation, with low numbers in the boxes. Occasionally there is more than one species in a box (the “beehive frames” as it were, separate them). The big surprise is the use of one particular box by Soprano Pipistrelles (Pipistrellus pygmaeus) for breeding.
In 2017 at the May inspection there were already an estimated 60 bats present, so with a friend from the Bat Group we did an emergence count using infra-red lighting in June and counted 104 bats emerging, leaving some non-flying pups behind.
Such heavy usage takes its toll on the material used. I’d quickly found that with other boxes the very powdery “rice-cake” Styrofoam material is quickly destroyed by the bats, so I now only use the more dense polyurethane foam in its many guises. So already by 2016 I had hung a replacement for this box beside it. But until the bats give it up I can’t take it down. And they were very reluctant to give up their slum. I wasn’t able to take it down until October 2018
Box AWC after 9 years service. It was almost completely hollowed out inside.
But now the scene changes again. Already in 2018 there were a couple of Noctules in with 7 Pips in October. Because they are separated by the panels of insulation they appear to live happily together. By October 2019 there were circa 100 Soprano Pipistrelles and 2 Noctules in this box. In March there was 1 Pip. But by May (no regular boxes inspection, just a lone visit) there were no Pips, but about 20 Noctules. By late June this had gone up to circa 30. So it looked like a take-over by the Noctules.
I’d passed this news on to a friend who was anxious to try to get some photographs of Noctules leaving a roost, and through this we eventually reckoned there were about 50 bats in the colony, but this included an unknown number of this year’s pups.
So what of the Soprano Pipistrelles? They appear to have de-camped to another box further down the valley. And a week later all the Noctules have left the breeding site! So I’ll have to watch this space, literally.
Count this lot! Noctules in a breeding colony.
See also:
https://northcornwallnaturalist.blogspot.com/
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