Internal Surveys

◆ Anyone entering an abandoned mine must

have appropriate training and experience.

◆ When properly conducted, internal surveys

are the most reliable and least labor-intensive type of survey for evaluating roost 

presence and quality. 

◆ Internal surveys allow better-informed 

decisions than external observations in

choosing appropriate mine-closure strategies.

Cold Season Surveys

◆ Unless pre-hibernation swarming (when

bats first appear at hibernation sites) is witnessed, internal surveys are the only way to

detect hibernating bats. 

◆ Hibernating bats must be identified with

minimal disturbance. Repeated arousal

causes bats to use fat reserves they need to

survive the winter.

◆ Inspecting even tiny cracks and crevices may

berequired to find hibernating bats. Guano,

roost staining, bat remains, and insect parts

indicate use during other seasons.

Recommendations

Warm Season Surveys

◆ The timing of warm season surveys varies

by local climate, but they are generally conducted during May through June in southern states and July through August in the

north. 

◆ Mine must be explored quietly because

many bat species cannot tolerate disturbances at roost sites, especially when they

are giving birth or caring for pups.

◆ Maternity colonies may use multiple roosts

during a single season, so do not assume

that because bats are not present at one

roost, a mine is not being used.

◆ Guano containing discarded insect remains

can indicate night roosting, even if no bats

are seen in daytime surveys. If night roosting is suspected, the mine can be checked at

night to observe the species and numbers.

◆ Identifying other kinds of use, such as

courtship or migratory stopovers, can be difficult, but multiple visits to the site or using

drop cloths (a light-colored sheet or piece of

plastic) to collect guano may help clarify use.

Townsend’s big-eared bats

(Corynorhinus townsendii) are 

frequent occupants of abandoned

mines throughout the western

United States and rely on them

year-round.

© MERLIN D. TUTTLE, BCI \ 849-5201
Background 

Abandoned underground mines 

provide important roosting habitat 

for more than half of the 45 bat 

species in the United States. And 

their use of abandoned mines is as 

complex as the varied environments 

the mines provide. Bats use mines for 

rearing young in the summer, hibernating, gathering for social activities 

such as courtship and mating, night roosting, and for crucial rest stops 

during spring and fall migrations. 

The process of determining whether 

bats are using a specific mine is not 

simple, but it can be accomplished reliably by following these recommendations. 

The assessment begins with a preliminary survey to describe all mine 

openings and record all information 

that can be gathered at each opening 

without underground entry. These 

data should include: entrance dimensions; elevation relative to other openings; airflow direction and temperature; ambient air temperature; obstacles such as rocks, vegetation, limbs, 

trash, portal or headframe timbers in 

the opening; potential hazards; estimated vertical or horizontal depth; 

presence of internal complexity, such 

as drifts, crosscuts, raises, winzes, or 

stopes; and observations of any 

wildlife or wildlife sign. If a mine 

cannot be eliminated as wildlife habitat by the initial survey, an external 

and/or internal survey is warranted. 

survey data at an abandoned m

shaft in California’s Joshua Tr

National Park. 

© SHERYL DUCUMMON, BCI \ 916-5406 
Abandoned underground mines

provide important roosting habitat

for more than half of the 45 bat

species in the United States. And

their use of abandoned mines is as

complex as the varied environments

the mines provide. Bats use mines for

rearing young in the summer, hibernating, gathering for social activities

such as courtship and mating, night roosting, and for crucial rest stops

during spring and fall migrations.

The process of determining whether

bats are using a specific mine is not

simple, but it can be accomplished reliably by following these recommendations.

The assessment begins with a preliminary survey to describe all mine

openings and record all information

that can be gathered at each opening

without underground entry. These

data should include: entrance dimensions; elevation relative to other openings; airflow direction and temperature; ambient air temperature; obstacles such as rocks, vegetation, limbs,

trash, portal or headframe timbers in

the opening; potential hazards; estimated vertical or horizontal depth;

presence of internal complexity, such

as drifts, crosscuts, raises, winzes, or

stopes; and observations of any

wildlife or wildlife sign. If a mine

cannot be eliminated as wildlife habitat by the initial survey, an external

and/or internal survey is warranted.

Background 

survey data at an abandoned m

shaft in California’s Joshua Tre

National Park.

© SHERYL DUCUMMON, BCI \ 916-5406
Abandoned underground mines

provide important roosting habitat

for more than half of the 45 bat

species in the United States. And

their use of abandoned mines is as

complex as the varied environments

the mines provide. Bats use mines for

rearing young in the summer, hibernating, gathering for social activities

such as courtship and mating, night roosting, and for crucial rest stops

during spring and fall migrations.

The process of determining whether

bats are using a specific mine is not

simple, but it can be accomplished reliably by following these recommendations.

The assessment begins with a preliminary survey to describe all mine

openings and record all information

that can be gathered at each opening

without underground entry. These

data should include: entrance dimensions; elevation relative to other openings; airflow direction and temperature; ambient air temperature; obstacles such as rocks, vegetation, limbs,

trash, portal or headframe timbers in

the opening; potential hazards; estimated vertical or horizontal depth;

presence of internal complexity, such

as drifts, crosscuts, raises, winzes, or

stopes; and observations of any

wildlife or wildlife sign. If a mine

cannot be eliminated as wildlife habitat by the initial survey, an external

and/or internal survey is warranted.

Background

survey data at an abandone

shaft in California’s Joshua 

National Park.

© SHERYL DUCUMMON, BCI \ 916-5406
Recommendations 

Internal Surveys 

◆ Anyone entering an abandoned mine must 

have appropriate training and experience. 

◆ When properly conducted, internal surveys 

are the most reliable and least labor-intensive type of survey for evaluating roost 

presence and quality. 

◆ Internal surveys allow better-informed 

decisions than external observations in 

choosing appropriate mine-closure strategies. 

Cold Season Surveys 

◆ Unless pre-hibernation swarming (when 

bats first appear at hibernation sites) is witnessed, internal surveys are the only way to 

detect hibernating bats. 

◆ Hibernating bats must be identified with 

minimal disturbance. Repeated arousal 

causes bats to use fat reserves they need to 

survive the winter. 

◆ Inspecting even tiny cracks and crevices may 

be required to find hibernating bats. Guano, 

roost staining, bat remains, and insect parts 

indicate use during other seasons. 

Warm Season Surveys 

◆ The timing of warm season surveys varies 

by local climate, but they are generally con

ducted during May through June in south

ern states and July through August in the 

north. 

◆ Mine must be explored quietly because 

many bat species cannot tolerate disturbances at roost sites, especially when they 

are giving birth or caring for pups. 

◆ Maternity colonies may use multiple roost

during a single season, so do not assume 

that because bats are not present at one 

roost, a mine is not being used. 

◆ Guano containing discarded insect remain

can indicate night roosting, even if no bat

are seen in daytime surveys. If night roosting is suspected, the mine can be checked 

night to observe the species and numbers. 

◆ Identifying other kinds of use, such as 

courtship or migratory stopovers, can be d

ficult, but multiple visits to the site or usin

drop cloths (a light-colored sheet or piece o

plastic) to collect guano may help clarify u

© MERLIN D. TUTTLE, BCI \ 849-5201 

Townsend’s big-eared bats 

(Corynorhinus townsendii) are 

frequent occupants of abandoned 

mines throughout the western 

United States and rely on them 

year-round. 
Internal Surveys

◆ Anyone entering an abandoned mine must

have appropriate training and experience.

◆ When properly conducted, internal surveys

are the most reliable and least labor-intensive type of survey for evaluating roost 

presence and quality. 

◆ Internal surveys allow better-informed 

decisions than external observations in

choosing appropriate mine-closure strategies.

Cold Season Surveys

◆ Unless pre-hibernation swarming (when

bats first appear at hibernation sites) is witnessed, internal surveys are the only way to

detect hibernating bats. 

◆ Hibernating bats must be identified with

minimal disturbance. Repeated arousal

causes bats to use fat reserves they need to

survive the winter.

◆ Inspecting even tiny cracks and crevices may

berequired to find hibernating bats. Guano,

roost staining, bat remains, and insect parts

indicate use during other seasons.

Recommendations

Warm Season Surveys

◆ The timing of warm season surveys varies

by local climate, but they are generally conducted during May through June in southern states and July through August in the

north. 

◆ Mine must be explored quietly because

many bat species cannot tolerate disturbances at roost sites, especially when they

are giving birth or caring for pups.

◆ Maternity colonies may use multiple roosts

during a single season, so do not assume

that because bats are not present at one

roost, a mine is not being used.

◆ Guano containing discarded insect remains

can indicate night roosting, even if no bats

are seen in daytime surveys. If night roosting is suspected, the mine can be checked at

night to observe the species and numbers.

◆ Identifying other kinds of use, such as

courtship or migratory stopovers, can be difficult, but multiple visits to the site or using

drop cloths (a light-colored sheet or piece of

plastic) to collect guano may help clarify use.

Townsend’s big-eared bats

(Corynorhinus townsendii) are 

frequent occupants of abandoned

mines throughout the western

United States and rely on them

year-round.

© MERLIN D. TUTTLE, BCI \ 849-5201