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Nordquist, G.E. 1983. Summary of two field trips (4-5 May and 17-19 August, 1983) to investigate the occurrence of bats at the potlatch plant at Cook, MN. Final report submitted to the Minnesota Department of Natural Resources. 11 pp.


The Potlatch oxboard plant outside Cook, Minnesota, has been the site of major bat accumulations during the late summer-fall of 1982 and again this year. Based upon reports of plant employees, the bats first appeared over a weekend in August, 1982. Several thousand bats formed huge clusters about the outside of the buildings, clung thickly along door edges, and grouped in large aggregations inside on the upper corners of the walls and ceilings. Clusters of bats were observed hanging outside through October and a few small clusters remained indoors until February, where they appeared to die one-by-one and drop from their roost sites. By March, 1983, when the plant began operation, no bats were present. However, approximately the same time this year, significant numbers of bats arrived at the plant.

The May trip was primarily exploratory, checking the region for indication of bat abundances and attempting to determine whether the 1982 bat influx was an isolated, chance event of whether it could be expected to occur again this year. The August trip was made in response to the reoccurrence of bats at the Potlatch Plant. Most of the work was concentrated at the plant site, however, the area surrounding the plant and the Tower-Soudan Mine were also investigated.

The number of bats at the Potlatch plant during the second visit exceeded 3000 individuals. Two species of bat were found at the plant. The most common was the little brown bat (Myotis lucifugus) interspersed with Keen's bats (M. keeni). Sexual composition of bats at Potlatch suggest it was an influx of migrating individuals (equal male and female numbers). Many of the bats acted sluggish, appeared to roost in inappropriate areas and were unusually heavily infested with ectoparasites.

The re-appearance of bats at the Potlatch Plant lays to rest any doubts that this is a recurrent phenomenon. Further study should be pursued. The Tower-Soudan mine, admittedly an aberration by its presence in an area with no naturally formed caves, may play an important role in the biology of at least two bat species, Myotis lucifugus and Myotis keenii. The nearly constant temperatures and high humidity make it an ideal hibernaculum for overwintering bats. The presence of ore dust on the wings of bats from Tower-Soudan and from the Potlatch plant suggest a close connection between these two sites and further underline the need for study in this region.