Problem Wildlife
Bats – Wildlife Fact Sheet #1

Bats in Buildings

In Newfoundland, the bat species usually found in buildings is the Little Brown Bat. This is the only common species found throughout the Island, and is also the most frequently encountered bat species found in buildings throughout its range.

Bats will occupy unused attics and eaves as roosts, and on occasion large breeding populations, known as nursery colonies, may congregate in such locations. Since bats are nocturnal, their presence may go unnoticed by the home-owner until unusual noises and possibly odours are evident. If a large bat colony is allowed to remain at large in a building, an odour problem will eventually result from the accumulated guano (droppings). Despite the fact that this is excellent fertilizer, it’s presence in an attic is not recommended! Considering that Little Brown bats can live for thirty years and habituate the same roost, potential exists for a sizeable collection of guano over time.

It is strongly advised that no action to evict bats from buildings be undertaken in early summer. This covers the period from about June 15 to August 15. We have recorded young born as early as June 22. It is thought that young bats are capable of flight at an age of around three weeks. However, young are not proficient fliers and insect-hunters at that age. Consequently, any action taken to seal off the bat colony in early summer may result in the stranding, and consequent death of any sealed in animals. These dead animals, as they decompose, may also result in unpleasant odours in the building. It is for these reasons that it is recommended that colonies not be evicted until late summer.

Prior to evicting a bat colony, consideration should be given to providing an alternate roost site. Plans are available for bat houses. These plywood and screen wafer-like structures can accommodate most colonies. One or more may be attached to the external surface of the occupied building, or located on a nearby outbuilding, pole or tree. Given the bat’s ability of consuming large quantities of flying insects around a home, there is good reason to have them nearby.

It is strongly recommended that exclusion techniques not be employed until late August. Any such action taken is meant to encourage the bats to leave and then prevent them from recolonizing. Exclusion techniques that can be employed include installing a light in the eave or occupied location or installing a loud radio in the same, often in combination with the light. If this is not possible, placing a quantity of mothballs in the bat living quarters is usually effective. Any exclusion attempt must be followed by a sealing of all access points. This may require caulking around chimneys and eaves. In many cases, narrow seams may be covered with a strip of nylon window screening (similar to that on vinyl slider window screens). This fabric is cut to a size that is around an inch longer and wider than the seam. The screen is attached by stapling it over the hole or seam along the top edge only. Because this lies flat over the outside of the seam, bats can push out through to exit the house, but cannot return.

If you are unsuccessful in evicting the bats in late summer, wait until late fall at which time suspected entry points may be sealed. Most bats will not overwinter in houses. They require a winter roost of high humidity, such as underground caves and mineshafts, since they will otherwise dehydrate and dessicate in less humid sites. By November, bats should have vacated the premises and action to seal up the access points can be undertaken at this time.

When attempting to clean bat (or bird and rodent) droppings from an attic or eave, first dampen down the site with water and bleach. Wear disposable gloves and a disposable face mask for your own protection. Accumulations of droppings in a warm environment may harbour histoplasmosis, a fungal disease to which we can be susceptible, and which is usually contracted by inhaling airborne spores.