Wisconsin State Natural Areas Program
Bay City Mine (No. 280)

Overview

Location

Pierce County.

Access

To protect sensitive resources, access is for research purposes only.

Description

Description

Bay City Mine features a regionally significant bat hibernaculum where more than 26,000 bats hibernate in winter. It is one of the more important hibernation sites in this area. The majority are little brown bats (Myotis lucifugus) but several hundred big brown bats (Eptesicus fuscus) and a number of eastern pipistrelle (Pipistrellus subflavus) and northern myotis (M. septentrionalis) are also found in the mine. In areas where present, bats tend to occur in high densities, often in large clusters. Composed of large sandstone chambers and with ceilings of over 50 feet, the numerous tunnels contain a variety of habitats - each with different temperatures, humidity, and wind currents. This microclimate diversity has contributed to the successful colonization of different bat species within the mine. The mine slopes down from the entrances, making the main passages appreciably colder than side branches during the winter. Unlike many caves, large clusters of big brown bats are found deep in the mine where cold air sinks to a low, dead-end passage. The isolated sections of the old mine are much warmer and less drafty and support a number of pipistrelle. Most of the mine is dry, except for localized sections of water seepage. Bay City Mine was donated to the DNR by the Unimin Corporation and was designated a State Natural Area in 1994.