United States Department of Agriculture

Forest 

Service

Research Paper 

NRS-27

Northern 

Research Station January 2015

Internal Cavity Characteristics of 

Northern Long-eared Bat 

(Myotis septentrionalis) 

Maternity Day-roosts

Alexander Silvis

R. Edward Thomas

W. Mark Ford

Eric R. Britzke

Meryl J. Friedrich
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Abstract

We report characteristics of seven tree cavities used as day-roosts by female northern 

long-eared bats (Myotis septentrionalis) during the maternity season in a deciduous forest 

in north-central Kentucky. Understanding the characteristics of cavities selected by bats 

will help us better understand the ecology of cavity roosting bats and the tree species 

and condition necessary for providing day-roost habitat. Cavity openings were created by 

either fungal decay or primary excavators. Length and volume of cavities were positively 

related to number of entrances. Mean area of entrances was positively related to the 

proportion of entrances created by primary excavators.

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Quality Assurance

This publication conforms to the Northern Research Station’s Quality Assurance 

Implementation Plan which requires technical and policy review for all scientifi c 

publications produced or funded by the Station. The process included a blind technical 

review by at least two reviewers, who were selected by the Assistant Director for 

Research and unknown to the author. This review policy promotes the Forest Service 

guiding principles of using the best scientifi c knowledge, striving for quality and 

excellence, maintaining high ethical and professional standards, and being responsible 

and accountable for what we do.

Cover Photo

A white oak (Quercus alba) snag provides roosting opportunities for female northern longeared bats on the Fort Knox Military Reservation, Kentucky, USA. Photo by Alexander 

Silvis, Virginia Tech, used with permission.
1

Th e Authors

ALEXANDER SILVIS and MERYL 

J. FRIEDRICH are post-doctoral 

researcher and graduate student, 

respectively, at the Department of Fish 

and Wildlife Conservation, Virginia 

Polytechnic Institute and State 

University, Blacksburg, VA.

R. EDWARD THOMAS is a research 

computer specialist with the U.S. 

Forest Service, Northern Research 

Station, in Princeton, WV.

W. MARK FORD is a research 

wildlife biologist and unit leader with 

U.S. Geological Survey, Virginia 

Cooperative Fish and Wildlife Research 

Unit, Blacksburg, VA.

ERIC R. BRITZKE is a rese archer 

with the U.S. Army Engineer 

Research and Development Center, 

Environmental Lab, Vicksburg, MS.

Alexander Silvis can be contacted at: 

silvis@vt.edu.

INTRODUCTION

Preservation of day-roosting habitat has been a focus of bat 

conservation in forested ecosystems and considerable eff ort has 

been undertaken to understand habitat selection at scales ranging 

from individual trees to whole landscapes (Brooks and Ford 

2006, Kalcounis-Rüppell et al. 2005, Perry et al. 2007). Bats 

select roosts based upon environmental constraints that aff ect 

physiology, and habitats used by tree-roosting bats vary widely 

among species and types of forests (Grindal et al. 1992, Lourenço 

and Palmeirim 2004) and sociality (Chaverri et al. 2007). Most 

studies on selection of day-roosts by myotine bats have focused 

on characteristics of the external (tree) and surrounding forest 

(Kalcounis-Rüppell et al. 2005, Miller et al. 2003). Although such 

metrics may be useful for understanding selection of habitat and for 

identifying suitable roost trees, they likely may not address the full 

set of characteristics bats may use to select roosts, particularly for 

cavity roosting species (Boyles 2007).

Many species of bats roost in cavities in live trees and snags (Kunz 

and Lumsden 2003). However, such cavities generally are not easily 

accessible to researchers, so data on their characteristics are limited 

(Parsons et al. 2003, Ruczyński and Bogdanowicz 2005, Sedgeley 

and O’Donnell 1999, Sedgeley 2001). In eastern North America, 

only the cavities used by Rafi nesque’s big-eared bats (Corynorhinus 

rafi nesquii) and the southeastern myotis (Myotis austroriparius) have 

been described in appreciable detail; both of these species roost 

in large bald cypress (Taxodium distichum) or water tupelo (Nyssa 

aquatica) trees with hollow boles that are relatively accessible by 

researchers (Gooding et al. 2004, Trousdale et al. 2008). Similarly, 

relationships between bats and fungi that decompose wood and 

primary excavators are poorly understood (Parsons et al. 2003). 

Although the sources of cavities used by bats have been recorded 

(Il’in 1998, Kalcounis and Brigham 1998, Psyllakis and Brigham 

2006, Vonhof and Gwilliam 2007), it is unclear how primary 

excavators and fungal decay interact to create potential roosts for 

bats (Jackson and Jackson 2004).

As part of a larger study on the social ecology of northern longeared bats (Myotis septentrionalis) in deciduous forests of central 

Kentucky (Silvis et al. 2012, 2014), we had the opportunity to 

remove and examine day-roosts that were used during the previous 

maternity season. In this report, we describe cavity characteristics 

and document cavity origin and volume.
2

MATERIALS AND METHODS

The study was conducted on the Fort Knox military 

reservation in Meade, Bullitt, and Hardin Counties, 

Kentucky. The forest cover on the reservation is 

predominantly a western mixed-mesophytic association 

(Braun 1950), with second-growth and third-growth 

forests dominated by white oak (Quercus alba), black 

oak (Q. velutina), chinkapin oak (Q. muehlenbergii), 

shagbark hickory (Carya ovata), yellow-poplar 

(Liriodendron tulipifera), white ash (Fraxinus 

americana), and American beech (Fagus grandifolia) 

in the overstory. Sassafras (Sassafras albidum), 

redbud (Cercis canadensis), and sugar maple (Acer 

saccharum) dominate the understory (Cranfi ll 1991). 

We captured northern long-eared bats between May 

and August 2011. We attached LB-2 radiotransmitters 

(0.31 g; Holohil Systems Ltd., Woodlawn, ON, 

Canada) between the scapulae of female northern 

bats using Perma-Type surgical cement (PermaType Company Inc., Plainville, CT, USA). Bats 

were released at net sites after a few minutes of 

capture. Methods followed the guidelines of Virginia 

Polytechnic Institute and State University Institutional 

Animal Care and Use Committee permit 11-040-FIW.

Using TRX-1000S receivers and folding threeelement Yagi antennas (Wildlife Materials Inc., 

Carbondale, IL), we located day-roosts daily for the 

life of the transmitter, or until the unit dropped 

from the bat. We uniquely marked each female 

northern long-eared bat day-roost and recorded 

species of tree; diameter at breast height (d.b.h.); 

height; and crown class (Nyland 1996; i.e., 1 = 

suppressed, 2 = intermediate, 3 = codominant, 4 = 

dominant), and decay class (Cline et al. 1980; e.g., 1 

= live, 2 = declining, 3 = recent dead, 4 = loose bark, 

5 = no bark, 6 = broken top, 7 = broken bole). We 

also visually estimated percent of remaining bark.

During the winter of 2011-2012 when bats were 

hibernating and trees were not occupied by bats, 

we felled 14 day-roosts from the previous maternity 

season and selectively extracted the portion of the bole 

that contained cavities. Day-roosts were selected to 

meet the needs of a concurrent study on sociality and 

represent highly used roosts. We restricted our analyses 

to descriptive statistics because of our small sample 

size. Cavities used as roosts were classifi ed following the 

method of Sedgeley and O’Donnell (1999) as knothole 

in large branch, knothole in trunk, trunk hollow, or basal 

hollow. Day-roost cavities were accessed by splitting boles 

along their axis using a band saw. We measured cavity 

entrance area using the formula for the area of an ellipse, 

A = πdh. We measured cavity volume by incrementally 

adding known quantities of water to each side of the bole 

and recording the total fi nal volume added to the cavity. 

To prevent water from being absorbed into the tree bole 

or escaping through exterior entrances, cavities were fi rst 

lined with a thin, conformable plastic sheet. We also 

measured cavity length and derived cavity diameter from 

volume and length measurements using the equation for 

the volume of a cylinder, V = πr 2

h. We visually assessed 

the relationships among sets of cavity characteristics by 

plotting variable pairs. All analyses were performed using 

the R statistical program (R. Development Core Team 

2013).

RESULTS

We captured 58 female northern long-eared bats and 

located 108 day-roosts. Of the 14 maternity day-roosts 

that were felled, only 7 had boles (6 sassafras and 1 black 

locust [Robinia pseudoacacia]) that remained intact after 

felling. Mean roost d.b.h. was 19.8 (± 1.5) cm; mean 

height was 13.1 (± 3.5) m. Bark covered an average of 

61.1 (± 42.3) percent of roost surfaces. All roosts were 

in suppressed canopy positions and were between decay 

stages 2 and 5. Overall, cavities had a mean (± standard 

deviation) volume of 1,069 (± 672) cm3

 and a mean 

length of 29.11 (± 16.7) cm. Average cavity diameter was 

6.90 (± 1.5) cm. Felled day-roosts contained a mean of 

3.43 (± 2.0) entrances with a mean entrance area of 61.5 

(± 63.9) cm2

 (Table 1). All cavity entrances were created 

by woodpeckers (54.6 ± 40.4%) or decay (45.4 ± 40.4%) 

at knotholes.

Several cavity characteristics appeared to be related 

among themselves. Length and volume of cavity 

increased with the number of entrances (Fig.1). Mean 

entrance area increased with the proportion of entrances 

created by woodpeckers (Fig. 2). All cavities showed 
3

Table 1.—Measurements of day-roost cavities used by female northern long-eared bat (Myotis septentrionalis) on the Fort Knox military reservation in Hardin, Bullitt, and Meade Counties, Kentucky, during 2011. All day-roosts contained only one cavity and were overtopped by larger trees in more dominant canopy positions. Sassafras (Sassafras albidum) = SAAL. Black locust (Robinia pseudoacacia) = ROPS. Mean entrance area reported is across 

all roost entrances. Roost Species

d.b.h. 

(cm)

Height 

(m)

Decay 

Class

Percent 

Bark Cavity Type

Cavity 

Volume 

(mL)

Cavity 

Length 

(cm)

Cavity 

Diameter 

(cm)

Ant 

Activity 

Level

Total 

Entrances

Woodpecker 

Entrances

Decay 

Entrances

Mean 

Entrance 

Area (cm2)

1 SAAL 28.9 15.3 4 45 trunk hollow 650 13.5 7.8 medium 2 1 1 11.32 SAAL 14.6 9.1 3 90 trunk hollow 540 29.5 4.8 high 4 0 4 14.63 SAAL 17.1 10.2 2 95 trunk hollow 405 15.5 5.8 low 2 2 0 171.04 SAAL 17.8 17.9 5 5 trunk hollow 1850 57 6.4 medium 5 4 1 59.35 ROPS 18.9 13.7 2 98 trunk hollow 2150 45 7.8 medium 7 6 1 129.86 SAAL 24.9 15.8 3 90 trunk hollow 910 29 6.3 medium 3 2 1 19.87 SAAL 16.2 9.6 5 5 knot hole in trunk 975 14.2 9.3 high 1 0 1 25.0

evidence of ant (family: Formicidae) activity through 

presence of serpentine galleries, usually within the 

bottom portion of the cavity (Fig. 3). Volume of cavity 

exhibited no apparent relationship with stage of roost 

decay or percent remaining bark.

DISCUSSION

Similar to the patterns of decay observed in other 

wildlife cavity trees (Zahner et al. 2012), the decay that 

we observed was confi ned largely to central and outer 

portions of the bole. However, because we were able to 

examine only those trees that did not break when felled, 

our results may be biased toward confi nement of decay 

in the central portion of the bole. For all trees examined, 

the remaining sapwood was remarkably sound and free 

of decay, despite the obvious presence of serpentine 

galleries created by ants. Contrary to our expectations, 

the relative level of ant activity did not appear to be 

related to volume of cavity. Although our use of plastic 

as a cavity liner prevented us from accounting for the 

volume of serpentine galleries created by ants, these 

galleries did not constitute “usable” space by bats. Usable 

space appeared to be unrelated to ant activity; however, 

the additional volume in these ant galleries may aff ect 

the thermal properties of the cavity (Burcham et al. 

2012). Additionally, it is possible that presence of ants 

may aff ect bats directly; antagonistic behavior between 

evening bats (Nycticeius humeralis) and carpenter ants (Camponotus fl oridanus) has been documented (Bender 

et al. 2009) although the direct interaction between bats 

and ants is largely unknown.

Of the cavity entrances we examined, approximately half 

were created by primary excavators, i.e., woodpeckers. 

Across forested ecosystems, the percentage of excavated 

cavities is indicative of the number of cavity trees within 

a forest stand. Th e percentage of excavated cavities versus 

cavities from other origins generally is high when the 

total number of cavities per hectare is small, and low 

when the number of cavities is high (Remm and Lõhmus 

2011). Used with current methods of estimating roost 

availability, recording the proportions of entrances of bat 

roosts created by primary excavators may help us better 

understand the availability of roosts for bats that roost in 

tree cavities.
4

Figure 1.—Relationship between the characteristics of seven cavities used as maternity day-roosts by 

northern long-eared bats (Myotis septentrionalis) in 2011on the Fort Knox military reservation in Hardin, 

Bullitt, and Meade Counties, Kentucky. A shows the relationship between cavity length and number of 

entrances. B shows the relationship between the number of entrances and cavity volume.

Figure 2.—Relationship between mean size 

of entrance and the proportion of entrances 

created by primary excavators for seven cavities 

used as maternity day-roosts by northern longeared bats (Myotis septentrionalis) in 2011 on 

the Fort Knox military reservation in Hardin, 

Bullitt, and Meade Counties, Kentucky.

Figure 3.—Representative cavity in a sassafras (Sassafras albidum) bole used as a maternity day-roost 

by northern long-eared bats (Myotis septentrionalis) on the Fort Knox military reservation in Hardin, 

Bullitt, and Meade Counties, Kentucky, during 2011. Each side of the scale on left is 66.5 cm.

1234567

20 30 40 50

Number of Entrances

Cavity Length (cm)

A

1234567

500 1000 1500 2000

Number of Entrances

Cavity Volume (cm3

)

B

0.0 0.2 0.4 0.6 0.8 1.0

50 100 150

Proportion of Entrances Created by Primary Excavators

Mean Entrance Size (cm2

)
5

We observed a trend of increased area of entrances 

with increased previous activity by primary excavators. 

Whether increased entrance area benefi ts northern 

long-eared bats or not is unclear, but increased entrance 

area may increase susceptibility to predators (Sparks et 

al. 2003) and roost competitors such as the southern 

fl ying squirrel (Glaucomys volans). Furthermore, area 

of entrances aff ects thermal conditions in the cavity 

(Coombs et al. 2010). Both factors have been observed 

to aff ect roost selection by other bat species (Lourenço 

and Palmeirim 2004, Sedgeley 2001, White and Jones 

2004). 

For cavity roosting bats, merely quantifying the external 

metrics of day-roosts fails to describe the total variation 

in day-roost resources used (Boyles 2007). Although tree 

cavities used by bats are diffi cult to measure, a better 

understanding of cavity microclimatic and physical 

characteristics is needed. Furthermore, increased 

understanding of the criteria bats use to select roosts, by 

improving eff orts to distinguish node and primary dayroosts (Johnson et al. 2012), may off er greater insight 

into non-random social assortment by bats (Kerth 2008, 

Rhodes 2007). If infl uences of cavity characteristics on 

suitability of day-roosts could be adequately described, 

it also may be possible to create artifi cial cavities in 

standing trees for species that rarely use bat boxes, such 

as the northern long-eared bat. Manual cavity creation is 

a management technique that has been used successfully 

with the endangered red-cockaded woodpecker (Picoides 

borealis) (Copeyon 1990). Additionally, it also may be 

possible to achieve similar results using less intensive 

arboricultural techniques by strategically wounding live 

trees (Smith 2006).

Our data provide the fi rst description of the 

characteristics of cavities used by northern long-eared 

bats, but should be interpreted cautiously because of 

our small sample size and variability among samples. 

Additionally, it is unclear how well our study sites on 

the Fort Knox military reservation represent forest 

conditions off base. We encourage further investigation 

of the characteristics of cavities used by bats.

ACKNOWLEDGMENTS

Th is research was supported by the U.S. Army 

Environmental Quality and Installation Basic Research 

6.1 program. We thank Jimmy Watkins, Mike 

Brandenberg, and Charlie Logsdon for their assistance 

in supporting this project. Th e Kentucky Department 

of Fish and Wildlife Resources graciously provided fi eld 

housing for this project.

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Printed on Recycled Paper

Silvis, Alexander; Thomas, R. Edward; Ford, W. Mark; Britzke, Eric R.; Friedrich, 

Meryl J. 2015. Internal cavity characteristics of northern long-eared bat 

(Myotis septentrionalis) maternity day-roosts. Research Paper NRS-27. 

Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern 

Research Station. 7 p.

This report discusses characteristics of seven tree cavities used as day-roosts by 

female northern long-eared bats (Myotis septentrionalis) during the maternity season 

in a deciduous forest in north-central Kentucky. Understanding the characteristics of 

cavities selected by bats will help us better understand the ecology of cavity roosting 

bats and the tree species and condition necessary for providing day-roost habitat. 

Cavity openings were created by either fungal decay or primary excavators. Length 

and volume of cavities were positively related to number of entrances. Mean area of 

entrances was positively related to the proportion of entrances created by primary 

excavators.

KEY WORDS: Myotis septentrionalis, cavity, roost characteristics, day-roost, 

northern long-eared bat, maternity, decay, roost formation, roost persistence

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