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Mark Kiser
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| Joined: | Tue Mar 12th, 2002 |
| Location: | Austin, Texas USA |
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Posted: Tue Mar 25th, 2003 10:00 pm |
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Here's the second excerpted article. Please let me know if you have any comments.
Thanks,
Mark
Bat Houses in Managed Forests of Southwest Oregon
Colin P. Dillingham, Stephen P. Cross, and Peter W. Dillingham
Roost sites are a critical resource for bats and may play a major role in determining population sizes and distributions.1 Forest-dwelling bats may be negatively impacted when natural roost sites such as exfoliating bark and tree crevices are lost. Bat houses, when designed and installed to meet local bat needs, could be a valuable tool to partially compensate for the loss of habitat resulting from timber harvest activities.
The objective of our study was to determine the orientation of bat houses that maximizes occupancy rates in managed forests. We tested two orientation variables: aspect and position relative to adjacent foliage. These variables and testing procedures should be considered when designing and implementing large bat house projects.
Methods
Our six study sites were located in the Siskiyou National Forest in southwestern Oregon. All sites were recently clear-cut forests dominated by Douglas fir, within 12.4 miles (20 kilometers) of each other, and at similar elevations (1,287 to 2,244 feet [390 to 680 meters]). The clear-cuts were approximately rectangular, with sides parallel to the cardinal compass directions. Sites were harvested between 1989 and 1990. Each site was 27.5 to 50 acres (11 to 20 hectares) and had 0.2 to 1.0 residual live trees/2.5 acres (1 hectare).
In 1991, eight bat houses were placed in each clear-cut. Four trees were selected on the edges of each clear-cut, one on each side of the clearing. Two bat houses were mounted on each tree, one facing the clear-cut and the other the adjacent forest. All houses were placed 10 feet (3.3 meters) above the ground on live, mature Douglas fir trees. Most placements were adjacent to mature or old growth forests, but some were adjacent to young (10- to 30-year-old) managed forests.
We used a small, single-chamber, wedge-shaped design. Each house was 24 inches tall and 12 inches wide (60 x 30 centimeters), with a bottom opening of 1 inch (2.5 centimeters) and tapering to zero inches at the top. The back of each house extended 2.4 inches (6 centimeters) below the opening to provide a landing surface. Houses were painted dark brown.
Bat houses were checked once in spring (February to April), once in summer (May to July) and once in fall (August to November) from 1991 to 1997 and monthly in 1998. No surveys were conducted during winter months. Occupancy was determined by shining a light up into the houses. Bats were not captured and were identified only as Myotis species (see box at right).
Results
Of 1,106 separate bat house observations, bats were recorded on 90 occasions for an 8.1% daily occupancy rate. However, 61% of all houses (29 of 48) were occupied at some point during the study and 83% of the south- and east-facing houses were used (20 of 24). Occupied houses contained one to eight individuals each. The daily occupancy rate was 10.1% (56 of 553 observations) for houses facing into clear-cuts and 6.1% (34 of 553 observations) for houses facing into forests. After adjusting for other variables, the odds of finding bats in houses facing the clear-cut were 1.9 times greater than for houses facing the forest. Additionally, occupancy was significantly higher in south- and east-facing houses than in north-facing houses.
Houses facing south were used the most, followed, in order, by houses facing east, west, and north (after adjusting for timber harvest area, time period, and orientation of bat houses into the adjacent forest; Table 1). Houses checked in spring were much less likely to be occupied than those surveyed in summer or fall. There was no difference in occupancy between summer and fall periods.
Discussion
Initial implementation of bat house projects should incorporate testing to optimize placement, particularly in forests. Our study included both shady (houses facing forests) and sunny (houses facing clear-cuts) conditions. We suggest that our south- and east-facing houses were preferred because they receive solar exposure in the morning, when houses are coolest, and shade in the afternoon, when overheating may be a concern. Based on our results, it appears that in coastal climates of the Pacific Northwest, heat conservation rather than heat load may be the limiting factor for roost orientation. Further research in other geographic areas would be helpful in determining the effects of solar radiation on bat house occupancy.
Radiotelemetry studies of forest-roosting bats in Oregon have shown that most roost sites are used on a temporary basis.2, 3, 4 The probability of a given site being used on a particular day is low and the presence of multiple sites may be important for reasons such as predator avoidance or microclimatic changes in the roost.5 This suggests that the low overall daily occupancy rate in our study is not surprising. As alternative roost sites are an important part of the normal life history of forest-dwelling bats,5, 6 this behavior should be taken into account when managing for these species.
This article was excerpted with permission from Northwestern Naturalist (Spring 2003), Vol. 84 (1). Colin Dillingham, a former a District Wildlife Biologist with the U.S. Forest Service in Oregon, is an ecologist with the U.S. Forest Service in California. Dr. Stephen Cross is a retired Professor of Biology at Southern Oregon University and designer of the Oregon Wedge bat house (see http://www.batcon.org/bridge/nbbp.html). Peter Dillingham is a statistician at Columbia Basin Research, School of Aquatic and Fisheries Sciences, at the University of Washington.
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1. Christy R.E. and S.D. West. 1993. Biology of bats in Douglas fir forests. In Biology and management of old-growth forests (Huff, M.H., R. M. Holthausen, and K.B. Aubry, technical coordinators). USDA Forest Service, PNW Research Station, Portland, OR. General Technical Report PNW-GTR-308.
2. Betts, B.J. 1998. Roosts used by maternity colonies of silver-haired bats in northeastern Oregon. Journal of Mammalogy, 79:643-650.
3. Ormsbee, P.C., W.C. McComb. 1998. Selection of day roosts by female long-legged Myotis in the central Oregon Cascade Range. Journal of Wildlife Management, 62:596-603.
4. Waldien, D.L., J.P. Hayes, and E.B. Arnett. 2000. Day-roosts of female long-eared Myotis in Western Oregon. Journal of Wildlife Management, 64:785-796.
5. Brigham, R.M. 1991. Flexibility in foraging and roosting behaviour by the big brown bat (Eptesicus fuscus). Canadian Journal of Zoology, 69:117-121.
6. Lewis, S.E. 1995. Roost fidelity of bats: a review. Journal of Mammalogy, 76:481-496.
Figure 1 caption: Study design showing arrangement of bat houses around clear-cuts. The daily occupancy rate was highest in bat houses with southern (11.6%) and eastern (9.7%) aspects. Significant differences in occupancy were found between houses facing south and those facing north and between those facing east and north. Houses facing into clear-cuts were used more often than those facing into forests.
Text box: Myotis bats native to this study area include the California Myotis (Myotis californicus), fringed myotis (M. thysanodes), long-legged myotis (M. volans), long-eared myotis (M. evotis), little brown myotis, and Yuma myotis (M. yumanensis). The first three species have not been documented using bat houses but are potential candidates. The long-eared myotis, long-legged myotis, fringed myotis, and Yuma myotis are species of special concern in the U.S. The fringed myotis is a species of special concern in Canada.
Editor?s Note: While BCI generally discourages mounting bat houses on trees, some species of forest-dwelling bats may benefit from this technique. As this study shows, south- and east-facing sites receiving sun are most likely to be successful. Install multiple houses around clearings on large diameter trees with branches starting 30 feet or more off the ground. Predator guards such as metal collars should be attached to trees four feet or higher above ground to deter rat snakes, raccoons, and other climbing predators.
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Dave Miller
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| Joined: | Tue Jun 4th, 2002 |
| Location: | Washington USA |
| Posts: | 67 |
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Posted: Thu Apr 3rd, 2003 12:29 am |
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Hi Mark,
This article is great the way it is.
I was amazed that their occupancy rate was as high as it was. I had the impression that bats can find plenty of roosts in a mature forest, thus generally ignoring bat houses there. I'm wondering if the clearcut caused an increase in the insect population thus making the clearcut area attractive to bats. This "edge effect" has greatly increased the deer population in Oregon (vs. deer populations in old growth forests).
I didn't see anything in the article about distance to water, I would think that would play a bigger role than facing the clearcut or the forest.
Dave
P.S. I grew up in a small town on the Oregon coast and now live just across the Columbia river in Washington, so I am familiar with clearcuts.
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