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Adding Bat Boxes to Our Green Roof

Green roofs give you all sorts of opportunities to play around and experiment with nature on your building. We've wanted to add bat boxes to our Sun Alta green roof for a while. If you're thinking about doing this yourself, now is the time to get started. Aim to have your bat box up before early May when bats come out of hibernation and return to their roosts.

The popular view of bats as pests might have you cringing at the thought of having bats in your backyard. They don't normally bite humans. Of the 1,240 bat species in the world, only three feed on blood and none of these are native to Canada. Bats pretty well leave humans alone and are great for eating mosquitoes and other insects. According to the Wisconsin Bat Monitoring Program, a million little brown bats will eat six hundred and ninety-four tons of insects per year! Take that Deet! There's real value attached to this natural pest control: it's estimated that a farmer would have to spend $4,000 to $5,000/yr on pesticides just to replace the pest control that the bats provide naturally.

Aside from their usefulness, bats are fascinating. If you are interested in learning more about bat species, check out this fun list of bat facts.

Back to the bat boxes. We're setting up a home for the little brown bat which are year-round residents in southern Alberta. This species should be more attracted to our bat box because they are more comfortable being around humans and more likely to make their roost in man-made structures like attics and barns. The little brown bat is a secure species in Alberta but still faces habitat loss. Putting a bat box in your property can help support their activities in the urban environment. More information about little brown bats at the bottom of this post.

Bat boxes are pretty simple to make. The Government of Alberta has published a guide of how to build your own bat box, though our pictures below might help you to visualize the finished product. There are also a number of places to purchase them online.

Looking at these pictures you might think the box seems narrow, but they're perfect for a little brown bat which will weigh no more than half an ounce. Inside the box, the bats snuggle together for warmth. The screen on the inside of the box helps to give the bats something to cling to so they can move around.

Before we put up the bat boxes, we used a dark wood stain on the outside of the box. It is also good to seal the joints with caulking. Bats love warmth. The bat box is like a little sauna for the bats. Nursing and caring for baby bats uses a tremendous amount of the mother bats' energy. Providing a nice warm shelter helps to reduce the amount of energy they need to keep warm. The optimal temperature range is between 30 and 40 degrees C.

When choosing a location for the bat box, there are a few things to keep in mind aside from full sun exposure. Your bat box should be located within half a kilometer of open water, like a river or lake. Water barrels and narrow basins will not do. Bats drink by flying low to the water and scooping it up with their mouths. They will not be able to do this with a water barrel and may drown if they try.

Your bat box also needs to be elevated at least three to four meters off the ground. Cats, raccoons and other animals are predators to bats, so remove and be aware of branches and other nearby objects these animals might use to climb to the box. If you are attaching the box to a wall, it could be helpful to attach a slippery plastic sheet around the box to stop predators from climbing.

Once your bat box is up it can take a couple years for bats to locate it and take up residence. Research shows that about 50% of urban bat boxes are occupied, so you have a decent shot of eventually getting some bats. 90% of occupied boxes become occupied within the first two years. So if you still don't have bats after two years, consider moving the box to a new location. Bat Conservancy International suggests installing your bat boxes on buildings or poles as they are easier for bats to locate: they're typically occupied two and a half times faster than those mounted on trees.

You can check if bats are using your bat box by shining a flashlight up the bottom of the box and taking a peak, though the bats may be nestled pretty far up inside. Perhaps a better way to check is to look for small black dots under the box: guano. A potted plant is recommended below the bat box to catch droppings (and fertilize your plant) and to provide a soft landing if any baby bats fall.

Finally, if bats do take up residence in your bat house, the Alberta Bat Action Team is collecting data on bat houses in Alberta. Please make sure to complete and submit the bat box data reporting form.

Facts on little brown bats:

  • The little brown bat can be found across much of North America, including all of the provinces and territories in Canada except Nunavut.

  • It is the most common species of bats in Canada. The little brown bat is classified as a secure species by the Government of Alberta.

  • Adults are typically 2.4–3.9" long and can have a wingspan of up to 11". At most, a little brown bat weighs no more than half an ounce. Females are typically larger than the males.

  • Adult little brown bats can eat as many as 300-3000 insects in a night. Nursing females can consume up to 110% of their body weight in a feeding period.

  • Summer roosting colonies are made up almost entirely of pregnant/nursing females and their young. These colonies can have anywhere from three to 1100 bats. Males are more mysterious: little is known about what they do with their summers or where they go!

  • Baby little brown bats are born blind and hairless and are completely reliant on their mothers for the first three weeks of their lives - but they grow up fast! At three weeks, they learn to fly. They are completely self-sufficient at one month old.

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