Although Batman had the “bat cave”, the truth is that most
bats in North America don’t use caves for summer roosting. There are a handful
of spectacular exceptions, especially in the southern USA (such as Bracken
Cave, Texas), and in British Columbia, Townsend’s Big-eared Bat has been
found to use caves in summer, but generally caves tend to be too cold to
support colonies of females and their growing young (known as maternity
colonies). Maternity colonies and nursery colonies represent the largest
aggregations of bats that we find in summer. Caves (and sometimes underground
mine sites) are important though. In Canada, they most often represent critical
habitat for over-wintering bats who use them for hibernation. Stable cold
temperatures and high relative humidity helps them get through a long winter hibernation
period when it is too cold to find any flying insects to eat.
Natural Bat Roosts
So what DO bats use for summer-roosting in North America? The
most commonly used summer roost-type for bats is found in trees. Big trees,
large diameter trees, trees with holes, trees with peeling bark, live trees,
dead trees, accessible trees, trees of all different kinds of species but
usually the biggest of whatever trees are at a site. So for bats, it is
important to keep large, older-age class trees on the landscape, but it is also
important to make sure there are smaller green trees that are in place to
eventually replace the old ones (i.e., recruitment). Some types of trees, like
Western Red Cedar can get very old and almost completely hollow inside. These
hollow trees can stand for years in a forest and make fantastic roosts for some
species of bats. Not all tree species age this gracefully. Most trees will rot
out and fall over. Red Cedar seem to be able to stand in place for much longer,
making them an especially important tree species where it exists. Tree
planting, on large and small properties can benefit bats of all species.
Generally, if you are planting trees, plant native species.
 |
| Potential bat roost trees can be in coniferous or deciduous tree species. |
Radio-tagged bats have been tracked to trees in studies
across North America in various forest-types. The general pattern of use is
similar with bats of various species favouring older, larger diameter (generally
greater than 30 centimetres diameter at breast height), taller trees with
moderate to slight decay in older stands. There may be some differences between
types of trees used by males or female bats or between roosts used by pregnant
or lactating females. There is some evidence that suggests that smaller bat
species may use spaces under loose bark more often that larger bat species
which may use tree cavities more often. Tree species used varies depending on
the forest type, however studies have found that bats may prefer trees on
south-facing slopes in dry habitats that exhibit features especially in upper
and midslope areas on the landscape.
 |
| Western Red Cedar used by Yuma Myotis on Cortes Island (photo: Christian Gronau) |
 |
| Inside of Western Red Cedar roost showing Yuma Myotis roosting (photo: Christian Gronau). |
If you have trees on your property, keep them. If they are big trees and maybe have a few dead limbs, get a professional arbourist to help you prune enough to make it safe, but leave as much peeling bark or spots with cracks and holes to allow for potential roost sites. If you have a big property and have a tree that needs to be taken down for safety reasons (maybe it is dead and the upper portion is unstable), cutting it so that you leave a really tall stump or a “stub” can potentially provide bats a place to roost. Some bat species have been known to use stumps, but if you are leaving a stub, try to keep at least a 3 metre tall length of the trunk standing. If the tree is dead but still stable, keep it intact. Other species like woodpeckers and owls will also make use of dead standing trees. The Wildlife Tree Committee of British Columbia have a number of publications that talk about the usefulness of dead standing trees and wildlife trees for many species of wildlife (including bats).
Rock roosts are also important. Bats in summer will use
cliff faces, rock crevices in rock outcrops and spaces under boulders
on talus slopes and just about any type of rock cranny they can find that
offers the right kind of temperature and protection. If you have rock habitat on your property, like
cliffs or caves, maybe take a look at the Best Management Practices for
activities around rock-type roosting habitat for bats (Chapter 3: Caving,Rockclimbing, Geocaching and other activities around caves and crevice habitatfor bats ). Essentially, for bats, protecting the rock feature from disturbance and from
damage or removal will help protect bat roosting habitat.
Property owners generally cannot modify their land to
provide winter habitat for bats, but if it already exists, this kind of habitat
can be protected from disturbance or degradation.
Bat Houses
Small property owners in urban areas might look at building
or buying a bat house. There is some fantastic information available on the Bat Conservation International webpage on bat
house design (with free design templates) and advice on whether or not to paint
your bat house (if you live in a cooler temperate area, consider painting it a
dark colour; if you live in an area with very hot summer temperatures, consider
painting it a light colour!), where to hang your house, how high it should be
and other details to pick the best site for your bat house. Bat houses require
their own discussion and lots of people are discussing them these days! Check
out Bat Conservation International’s resources for bat houses.
Bat houses range from small single-chambered houses, to multi-chamber
designs and a design known as a “rocket-box”. There are a lot of other designs
out there as well. In my opinion, you want something that is multi-chambered if
you are trying to offer a location for a colony of breeding females in summer.
Rocket-boxes are also an excellent design because they allow for a number of
bats to roost together,
but also allows individuals the ability to move around
in the roost so they can access a range of internal roost temperatures. If it
gets too hot, they can move down to the bottom of the roost, for more warmth,
they move to the top, inside the bat house to find the warmest spot. You need
to consider what kind of climate you have in your backyard when you pick your
bat-house design if you want bats to use it. Even better is to offer a number
of bat houses with different designs. This gives your bats a chance to change
positions depending on their energy needs.
 |
| Cross-section of a rocket box showing the central post and "layers" or chambers outside of the post. This is a demonstration model to show the interior of this type of bat box. Note the interior chamber walls have been "ridged" to provide bats with a surface to cling to with their little bat feet. |
Recently, there has been a surge of bat house building
activity across North America as concerned citizens become aware of the plight
of our bat species due to White-nose Syndrome. Populations of bats across North
America have been in decline for some time, but huge losses of large colonies
of hibernating bats in eastern North America has prompted listing at least
three once common bat species as endangered. Among my own bat biologist peers,
there has been some concern that “single-chambered” bat boxes might not be the
best design for bats (especially mothers with young). The small box design is
popular with builders but it doesn’t give bats the opportunity to use a variety
of microclimates within the roost. The small boxes, once painted black, can get
very hot when placed in a site that gets a lot of afternoon sun. Mother bats do
like a hot roost, but if it stays consistently well above 38C or more, it may
cause young bats to over-heat and die. More research needs to be done on bat
species’ roost temperature preferences and tolerances and the variation of
temperatures provided by different bat house designs. Multi-chambered bat house
plans and “rocket-box” designs that provide a variety of microclimates within a
single bat house roost and these bigger boxes may provide better bat roosting
habitat. Designs for multi-chamber boxes and for rocket-boxes are available on
the Bat Conservation International website.
If you are going to put up bat houses, I would recommend building
a couple of different kinds (multi-chambered and rocket box for example) and
give bats multiple boxes to choose from. Make sure they are at least 3 metres
off the ground, and in a position where it gets late afternoon sun. If you are
in a colder part of the country, paint the box a dark colour. It make take a
couple of summers for bats to find your bat house. These structures may also
require maintenance. Check them in the fall for wasp nests or loose boards.
Make sure the interior is roughed up or that you have added plastic screening
to the inside surface so bats have a good surface to hang from. Plastic
screening can break down over time, so you might have to replace it at some
point.
 |
| A simple rocket box design. |
 |
| Rocket box from below. |
Bat Roosts in Buildings
In summer, females raising young bats seek out very warm
roosts – attics of houses or other buildings with access (bats can squeeze into
spaces as wide as your pinky finger!) may be used as roosting sites for bats
however not all bat species will use buildings. Large colonies are most often
“maternity” colonies composed of pregnant or nursing mothers and their babies.
In summer, males use separate, cooler roosts and they may roost alone or in
small groups (usually less than 10). Maternity colonies can be quite large
(depending on species); in British Columbia colonies of 100-200 Little Brown
Myotis have been found using building attics, British Columbian Yuma Myotis
have been known to form very large colonies (of 2000 bats or more) in building
attic spaces. Big Brown Bats in Alberta have been found using buildings with
colonies ranging from less than 30 to the largest with 800 bats. Another common
“building user” is Townsend’s Big-eared Bat. In Canada, Townsend’s are only
found in British Columbia, ranging throughout southern and coastal regions with
records throughout the dry interior as far north as William’s Lake. This bat is
easily recognizable with huge ears and a habit of roosting in highly visible
open spaces (this is also one of my favourite bats with a sweet disposition and
amazing flight abilities – it can hover!). The largest colonies of Townsend’s
have been found in buildings in the B.C. interior and on the coast with numbers
ranging from 150-250 bats.
 |
| Townsend's Big-eared Bat roosting in a building in British Columbia. (Photo: Juliet Craig) |
Property owners in rural areas might survey any older
buildings they have on their property for bats. Bats can safely occupy
human-used buildings. In the United Kingdom, there is legislation that protects
bat roosts in buildings. Bats do not chew building materials or wires and they
do not bring in “nesting material”. Their impact in a building can be minimized
by ensuring that they cannot access human-living quarters and by setting up a
system so that you can annually or biannually clean up under roost sites to
remove accumulated guano (which makes excellent garden fertilizer because of
the high nitrogen content). Bats that favour the use of buildings seem to be
losing roosts as new building designs don’t necessarily accommodate bats the
way old buildings have done in the past.
Most often when bats
occupy a home attic space, the homeowner complaints are focused on the
accumulation of guano (bat poop) in the attic or wall space and/or noise of
little bat feet scrabbling around at dusk and evening. Bats can be accommodated
through modification of the roost space. Sealing openings between the
human-living space and bat-roosting space can eliminate intrusions of bats into
the home. Setting up thick plastic sheeting across the attic floor area to
catch falling guano can make annual clean-ups a simple task. A layer of
insulation under the plastic sheeting over the attic floor can eliminate noise
issues. With a little education and a bit of modification, bats can be safely
accommodated in attic spaces of human-occupied buildings. Homeowners should
ensure that their pets have pre-exposure rabies vaccinations (a reasonable
precaution even without the presence of bats in your attic); children and
occupants should be advised to never handle bats (a wise precaution for anyone)
and annual guano clean up should be conducted using a mask (to protect workers
from potential exposure to hanta virus from mouse droppings and/or
histoplasmosis, a condition that results from the inhalation of spores of a
fungus that can grow in the guano of bird or bat droppings in warm and humid
conditions – these would be the same precautions you would take if you were
cleaning up your chicken coop).
If you are going to exclude bats from your building (or
modify the roost area), consider setting up bat houses at least one full summer
before you begin an exclusion process. This gives bats time to scout out the
potential roosting habitat you have set up for them which increases the chances
that they will find your bat houses and use them. Exclusion should only be
initiated when you are sure there are no bats left in the building roost. For
most parts of North America with cold winter temperatures, this means very late
in the fall after the first frost and a series of evenings when nightly
temperatures are around or below freezing. Bats and their young are almost certainly out
of their summer roosts by this time. Almost all of our Canadian bat species
hibernate for the winter months. They leave their summer habitat and find an
appropriate wintering site (such as a cave or deep crevice). Young bats are
usually flying about a month after they are born, but may require another month
at the roost while they perfect their flight abilities, learn to hunt and gain
enough fat reserves to get them through the winter period. Timing of birth,
first flight of young bats and ultimately the dispersal of the summer colony
can vary from season to season and may vary significantly depending on your
local climate. Get to know your roost by counting the number of bats exiting
and noting the time when baby bats are born. It may take a year of observing
your colony before you can safely conduct a bat-friendly exclusion of your
resident bats.
Types of Bat Roosts
Bat roost types and the use of these roosts can vary
depending on the time of year, sex of the animal (males and females choose
dramatically different roost types in summer), reproductive condition (breeding
females choose much different roosts than males or non-breeding females),
species (we have 21 (possible) species of bats across Canada, some have very specific
roost preferences, other are generalists), and the type of activity involved.
The table below provides definitions for the various types of roosts that may
be used by bats:
Roost Type
|
Definition
|
Ephemeral roost
|
A bat roost in a feature where the
characteristics important to bats (e.g., microclimate) may change quickly
and/or unpredictably; for example, an area under sloughing tree bark.
|
Permanent roost
|
A roost that is available for
bat use over many years and has suitable characteristics (e.g., microclimate,
access) that remain stable over time. Examples of permanent roosts include
caves, cliffs, mines, bridges, buildings, and large hollow trees of a
slow-decaying species, such as western redcedar (Thuja plicata).
|
Night-roost
|
A roost where
bats rest at night between foraging bouts. Bats may roost singly or
congregate.
|
Day-roost
|
A roost where bats rest
during the day in spring/summer/autumn. Day-roost types include maternity
roosts, bachelor roosts, and mixed male/non-reproductive female/yearling
groups. Use of a specific day-roost may be seasonal or variable within a
season.
|
Maternity roost
|
A roost used outside the winter period by
adult females that are capable of reproduction.
|
Nursery roost
|
A roost where females congregate to give birth and
raise their young (adapted from Knight and Jones 2009). A nursery roost is a
type of maternity roost.
|
Bachelor roost
|
A roost used by
one or more males during the day.
|
Fall migratory rest stop
|
A
roost used by bats during migration between summer and winter habitats.
|
Winter hibernation roost
|
A site where one or more bats hibernate in
winter (hibernacula [plural]). A given hibernaculum may be used by bats for only
part of the winter, and may not be used every winter.
|
Permanent roosts should receive a
higher level of protection than ephemeral roosts and therefore should always be
a priority whenever there is development planning for an area. Hibernation
sites are considered critical habitat and are generally believed to be
relatively uncommon across most landscapes. Maternity roosts and nursery roosts
should also receive high levels of protection, especially during the periods
when they are occupied. All roost sites
are considered important, however further understanding of population
structures for various species and behavioural ecology of bats is needed to
understand just how important particular sites may be.
What is a significant bat roost?
Currently, bat biologists face
the question, what constitutes a “significant” bat roost site. The Best Management
Practices for Bats in British Columbia (BCMOE 2016) offers this definition:
Significant bat roost:
· any hibernaculum or swarming site;
· a roost used by a nursery colony of a Red- or Blue-listed species (any number of individuals), or a nursery roost used by more than six females of other species (can include mixed species groups);
· a roost used by a maternity colony of Red- or Blue-listed species (any number of bats), or a maternity roost used by more than four females of other species (can include mixed species groups);
· any permanent type day-roost used by a male or a non-reproductive female of a Red- or Blue-listed species, or > 10 males/non-reproductive females/juveniles of other species (can include mixed species groups);
· any night-roost used by a Red- or Blue-listed species or > 10 bats of other species (can include mixed species groups);
· any regularly used roost of a species listed under Schedule 1 of the Species at Risk Act (any number of individuals); or,
· any other roost deemed significant by an experienced bat biologist.
The “Red-list” includes any species or ecosystem that is at risk of being lost (extirpated, endangered or threatened); the Blue-list includes any species or ecosystem that is of special concern in British Columbia.
Summary
·
Retain older-aged trees on
your property; protect cliffs, caves, rock outcrops
·
If you have bats using an
out-building as a roost, try to find ways of retaining the bats and minimizing
conflict
·
If you need to exclude bats
from an attic, use bat-friendly techniques and conduct exclusions after young
bats are flying or after bats have left the roost for winter hibernation
elsewhere. Providing alternate roosting sites in nearby bat houses may improve
your chances of successfully relocating your bat colony out of your attic
space.
Useful links to sites with
bat house plans and advice on where and how to set up bat houses:
o
Canadian Wildlife
Federation – How to set up and build a bat house
(this site includes plans for “rocket boxes” which I think are excellent)
o
British Columbia Community
Bat Programs – Install a
bat house (with downloadable plans and advice for British Columbians)
o Alberta Community Bat Program – Got Bats?
o
British Columbia Best
Management Practices for Bats – Chapters 1-4 [Chapter 1: Introduction, Chapter 2: Mines,
Chapter 3: Caves and Crevices, Chapter 4: Wind Energy]
o Alberta Community Bat Program - Bat Houses in Alberta
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