1. ODOT Environmental Services November 15, 2000
Bats and ODOT Bridges
ODOT Environmental Services
November 15, 2000

2. ODOT bridges are used by bats for roosts
13 species of bats use bridges for roosts;
Bridge roosts have replaced tree roosts that have been cut;
ODOT bridges are important to bat conservation.
Bats use ODOT bridges for roosts, in all five ODOT Regions. There are 15 species of bats in Oregon and 13 species are known to use bridges. Bridges have been surveyed across most of western Oregon, including southwestern Oregon, the Willamette Valley, the Coast Range (BLM and ODF lands), and the central Cascades (USFS lands).
Why concern ourselves with bats? Evidence from bat surveys and inventories conducted in the last half century indicate that the western big-eared bat and all forest-dwelling bat species in Oregon are experiencing population declines. ODOT bridges actually make up for the loss of roosting sites in coniferous forests for all of the forest-dwelling bats in Oregon. Timber harvest in the last 40 years has reduced the number of roost sites in trees available to bats. So, ODOT bridges are an important source of roosting habitat for bats that contributes to the conservation of these beneficial animals.
Bats are good. They eat insect pests and are therefore beneficial to agriculture and forestry.

3. Basics of Bat Behavior Bats live year-round in Oregon;
Winter: some bats migrate; most hibernate (not in bridges) - some bats are active in western Oregon;
Summer: Day roosts (females in maternity colonies; males solitary or in bachelor colonies); Night roosts;
Emerge at dusk, drink, feed, rest at night roosts, feed again, go to day roost at dawn, sleep (in torpor) all day.
Bats live year round in Oregon. Four species are thought to migrate south and leave the state. The other 11 species spend the winter, most hibernating through the winter in hibernation roosts or hibernacula. Some bats west of the Cascades are active during the winter. Hibernation starts in October-November and lasts until April. Hibernation sites that we know of are mostly in caves and mines, none in bridges, though bats are found in buildings during the winter in western Oregon. Some western Oregon bridges are in use all year as night roosts because of the temperate climate close to the Pacific Ocean. Mating takes place in the fall, prior to hibernation, and the young are born in the spring following emergence from hibernation. Pregnant females gather together in maternity roosts in April or May where the young are born and raised. Males roost in different sites and are more solitary. Most maternity colonies start dispersing in late July as young begin flying. In August, most are in the process of migrating.
Bats sleep through the day in day roosts, which are located in dark, hidden places, such as tree cavities or caves. Bats leave their day roosts just as it is getting dark and fly immediately to a pond or stream for water. Then they fly around feeding for several hours, after which they fly to a night roost. The bats will hang out at the night roost for a while, then go feed some more. A bat will typically go to a night roost four times during the night between feeding flights. Then near dawn, the bat will feed and drink again, then return to a day roost where they will spend the day sleeping.

4. Night roosts and day roosts
Night roosts: Resting places to keep warm, digest food, engage in social behavior.
Day roosts: Hidden, dark places (crevices, caves, tree cavities) where the bats sleep through the day.
A night roost is any resting site occupied during the evening. Night roosts may be used just long enough to consume a prey item between foraging bouts, or span most of the night. Night roosts are not in the same locations as day roosts. Night roosts are located near feeding and drinking sites where the bat can find warmth and shelter. Bridges provide ideal night roosts because they are located over streams where the bats can eat and drink, they retain heat, and they provide vertical surfaces where the bats can hang. Stuart Perlmeter has found up to 500 night-roosting bats, representing as many as five species, under a single bridge in the western Cascades.
Bridges are also used by bats for day roosts in the summer, as maternity colonies for females and solitary or small group roosts for males. The day-roosting bats use crevices in all designs, and hidden caverns in box-beam designs. The Robertson Bridge in Grants Pass is used as a maternity roost for Yuma myotis. The Cottage Street Bridge over Bear Creek in Medford was being used as a maternity colony. The big-eared bats are unique in that they will day roost out in the open on vertical surfaces, such as under bridges. Stuart Perlmeter has found a few male western big-eared bats using bridges in the Cascades as day roosts. Bill Warncke found a big-eared bat during the day under the Thomas Creek Bridge.
So bats in Oregon are known to use bridges for day and night roosts but not for hibernation.

5. Bats select certain bridge types
Concrete bridges:
T-beam, Box beam, Sub-structure with vertical surfaces; Larger bridges;
Timber bridges:
Minor use if not treated with preservatives
Steel bridges not used
Certain types of bridges are used as roosts by bats. Concrete bridges are used the most, and the I-beam and box beam designs get the most use. Flat-bottomed precast slab bridges don’t provide as many vertical surfaces. Larger concrete bridges get more use as night roosts than smaller bridges.
Timber bridges get minor use, and where bats use timber bridges, the wooden stringers or beams are un-treated. Bats do not use wooden bridges where the timbers are treated with creosote, pentachlorophenol, chromated copper arsenate, or other wood preservatives. The available survey data indicate that about 3% of wooden bridges in the Willamette Valley get used by bats. The percentage use of timber bridges in southwestern Oregon may be higher. In fact, the maternity roost of Yuma myotis at the Robertson Bridge in Grants Pass is located between un-treated timber stringers. Timber bridges may not warm up as much as concrete bridges during the day, and the wood may lose heat faster than concrete at night. Steel bridges are not used at all by bats. However, don’t overlook the concrete abutments of steel bridges in they are arranged on the landscape in such a way so as to provide a cavern-like shelter.

6. Bridge location is important
Sunny locations important
Shaded bridges in trees, canyons, or gorges get little use
Solar radiation = warmth at night
Larger bridges absorb more heat
The location and orientation of the bridge in the landscape is an important factor in whether bats will use the bridge for roosting. The more sun (solar radiation) a bridge gets, the more heat it absorbs, and the more likely it will be used for roosting. Bridges in canyons or gorges, or bridges that are beneath the canopy of adjacent trees, will get little solar radiation during the day, and will not warm up. Bats won’t use a bridge that has not warmed up during the day. Bridges that are in open situations where the sun can shine on them all day warm up the most, and these bridges have the greatest potential for use. Bridges oriented in an east-west direction will get more uniform warming than bridges oriented in a north-south direction. Larger bridges absorb more solar radiation than smaller ones, and retain that head longer into the night. Bats appear to use bridges that span water more often than those that don't.

7. Bridges are used as day roosts and night roosts
Crevices
Inside box beams
Hollow spaces
Caverns
NIGHT ROOSTS:
Vertical surfaces
Ceilings
Abutments
Ends of the bridge
What parts of a bridge are used by bats? On concrete bridges, bats use crevices such as expansion joints if the crevice is at least 1/2 inch wide. Bats use the interior of box beams if they can get in. And for night roosts, bats use the vertical surfaces underneath the bridge on I-beam and other designs where vertical walls are provided. Pallid bats will hang from the ceiling. On vertical surfaces, the bat will position itself as far up as it can, up against the ceiling in the corner. The ends of the bridge near the abutments get the most use, probably because air movement underneath the bridge cools the mid-section of the bridge, so the end sections stay warmer longer. Bats avoid crevices directly over water, probably because the areas do not retain heat as well.

8. How to recognize bat use
You see bats. Look in crevices;
You hear bats. High-pitched chirps;
Bat sign: guano - dirty rice grains beneath vertical surfaces or crevices
Bat sign: urine stains - white and powdery on vertical surfaces;
Bat sign: body oil stains - dark, on vertical surfaces
You see bats. Since bridges are inspected during the day, look for them in crevices. They will not be out in plain sight on vertical surfaces during the day.
You hear bats. You can sometimes hear high pitched chirps. Of course, other small mammals like rodents make high pitched chirps also.
You see bat sign. Bats leave stains where they are in contact with the surface, formed by body oils, urine, and guano. Stains can be white, from crystallized urine, to dark, from body oil, or a mixture of urine and guano. Bat urine crystallizes at room temperature, appearing white and powdery where it has accumulated. Bat guano is typically dry and looks like dirty grains of rice. Bat dropping contain bits of undigested insects, are powdery when crushed, and are not white and chalky like bird droppings. A grain of bat guano is shiny and is made up of the exoskeleton or wings of insects. Guano grains will be dark brown, almost black, which are made up of skeletons of flying insects. Some bats feed primarily on moths. Their guano grains will be gray or light brown, from the wings of moths. Since bats hang on vertical surfaces, their guano will fall to the ground or whatever surface is directly beneath where they have been hanging. By comparison, rodent droppings will be made up mostly of vegetation. Slug droppings are ropey. Bird droppings typically are streaked with white and are larger.
SHOW SIMON WRAY’S BAT SIGN SLIDES

9. Bat Guano Contains insect parts, no vegetation;
Dark brown to black: insect skeletons;
Gray: moth wing scales;
Size: rice grain (small bats);
Size: puffed wheat (large bats)
PASS AROUND BAT GUANO SAMPLES
Bat guano is typically dry and looks like dirty grains of rice. Bat dropping contain bits of undigested insects, are powdery when crushed, and are not white and chalky like bird droppings. A grain of bat guano is shiny and is made up of the exoskeleton or wings of insects. Guano grains will be dark brown, almost black, which are made up of skeletons of flying insects. Some bats feed primarily on moths. Their guano grains will be gray or light brown, from the wings of moths. Pat Ormsbee is doing research on whether DNA from bat guano can be used to identify the bat.
Since bats hang on vertical surfaces, their guano will fall to the ground or whatever surface is directly beneath where they have been hanging.
By comparison, rodent droppings will be made up mostly of vegetation. Slug droppings are ropey. Bird droppings typically are streaked with white and are larger.

10. Safety risks Do not pick up bats;
Bats out in the open during the day are probably sick, and could be rabid; only rabid bats can transmit the disease;
Bats can get rabies but they do not carry the rabies virus;
If you need to move a bat, use a tool (shovel, broom);
Are bats a safety risk, to either humans or bridges? Bats are not dangerous to people. Bats do not carry the rabies virus, and only rabid bats can transmit the virus to humans. Rabid bats are sick, and may be found on the ground during the day. So, do not pick up bats, since they are probably sick, and may be rabid. If you need to move a bat, use a shovel or other tool. Regarding risks to our bridges, bats do not compromise the structural integrity of bridges. Ongoing studies in Texas have shown that bat colonies, even large ones, do not damage highway structures, and water sources under roosts are not negatively impacted.

11. Can bat presence affect your bridge work?
Yes, if the bridge has a maternity colony; the young cannot fly until late July; disturbance or demolition could kill the young bats.
Night roosting and day roosting by males is not a concern; they can find other roosts.
One of the reasons to look for bats at bridges is to coordinate bridge maintenance or other bridge work in such a way as to avoid or minimize disturbance, if possible.

12. What to do if you find bats
Contact the Region Environmental Coordinator (Richard Beck, Molly Cary or Brian Bauman, Max Mizejewski; Shelly Schmidt, or Chuck Howe);
Determine if maternity colony (biologist)
Coordinate activities April 1 - September 1.
SHOW SIMON WRAY’S RECOMMENDATIONS SLIDES

13. Big brown bat Photo by Merlin Tuttle
The big brown bat is a species that uses bridges a lot. Big browns use vertical surfaces under bridges for night roosting, and use recesses and crevices as bachelor colonies and maternity colonies.
Big brown bat
Photo by Merlin Tuttle

14. Roosting big brown bats.
Photo by Merlin Tuttle

15. Pallid bat Photo by Merlin Tuttle
Pallid bats use vertical surfaces of bridges for night roosting along the John Day River. Pallid bats also use alcoves, recesses, and expansion joints of bridges for bachelor colonies and maternity colonies.
Pallid bat
Photo by Merlin Tuttle

16. Pallid bat
Photo by Merlin Tuttle

17. Silver-haired bat
Photo by
Merlin Tuttle

18. Hoary bat
Photo by Merlin Tuttle

19. California myotis
Photo by Roger Barbour

20. Photos by Merlin Tuttle
Little brown myotis
Photos by Merlin Tuttle
Little brown myotis use bridges for night roosting in western Oregon. Little brown myotis use crevices as bachelor colonies and maternity colonies.

21. Photo by B. Moose Peterson
Long-eared myotis
Photo by B. Moose Peterson

22. Fringed myotis
Photo by Roger Barbour

23. Long-legged myotis
Photo by Roger Barbour

24. Yuma myotis
Photo by Merlin Tuttle

25. Small-footed myotis Photo by Merlin Tuttle
The small-footed myotis is one of the four species of bats in Oregon that only occurs in Regions 4 and 5. It uses vertical surfaces of bridges for night roosting and crevices as maternity colonies.
Small-footed myotis
Photo by Merlin Tuttle

26. Western pipistrelle
Photo by Merlin Tuttle

27. Brazilian free-tailed bat
The Mexican free-tailed bat occurs in southwestern Oregon (Region 3) and uses crevices in bridges as small bachelor colonies and large maternity colonies.
Brazilian free-tailed bat
Photo by Merlin Tuttle

28. Townsend’s big-eared bat
The Townsend’s big-eared bat is the most sensitive bat species in Oregon (it’s population is in decline). The big-eared bat is primarily a cave dweller, and eats mostly moths.
The big-eared bat uses cave-like sections of bridges. This bat has a maternity colony in the Service Creek bridge over the John Day River. The big-eared bat uses bridges for night roosting.
The big-eared bat is unique in that solitary males will use vertical surfaces out in the open under bridges for day roosting. Bill Warncke found a big-eared bat during the day on the abutment of the Thomas Creek Bridge in Curry County.
Townsend’s big-eared bat
Photo by Merlin Tuttle

29. Townsend’s big-eared bat
Photo by Merlin Tuttle