Guest blog: USFWS Converts Cold War Era Bunker to Bat Hibernacula in Northern Maine

What do you do with 43 cold war era bunkers in northern Maine? This has been an ongoing question within the U.S. Fish and Wildlife Service for nearly two decades. The bunkers are part of the former Loring Air Force Base. Loring Air Force Base’s Strategic Air Command was the country’s first operational nuclear weapons facility that provided storage and aerial delivery capabilities of nuclear warheads. When the base closed in 1994, much of the land surrounding the base (including the bunkers) was transferred to the U.S. Fish and Wildlife Service. Through this transfer of property, Aroostook National Wildlife Refuge was established. For years the grasslands surrounding the bunkers have provided exceptional grassland bird habitat, but refuge staff felt that the bunkers had more potential than merely providing nesting habitat for upland sandpiper and savannah sparrows on their grass covered roofs. The bunkers remained a curiosity for years and biological staff speculated that perhaps the structures could provide overwintering hibernacula for bats.

Installing thermal mat on roof of bunkerIn light of the devastating effects of white-nose syndrome on little brown bats in the Northeast, refuge staff discussed the feasibility of converting a few of the bunkers so that they could provide winter hibernacula for bats. In the winter of 2011/12, refuge staff examined the structural integrity of all 43 bunkers and ultimately decided on monitoring the temperature and humidity in two of the bunkers. The results of the preliminary data showed that the bunkers remained between 37-39 F° throughout the winter (even though outside temperatures dropped below -30F) and that relative humidity stayed above 90% (critical for hibernating bats).

Interior of bunker with roosting substrateIn the fall of 2012, U.S. Fish and Wildlife Service staff modified one of the bunkers to improve temperature regulation and increase relative humidity by adding a thermal mat and 18” of soil to the top of the bunker, and installing pools of water. Four solar powered motion activated infrared cameras were installed in the bunker to monitor the bats throughout the winter. Additional roosting substrates were placed in the bunker to permit the bats a greater diversity of objects to hang from (i.e. hollow log, bat house, wire mesh, plastic mesh and nylon netting).

Live video feed In late December of 2012, 30 male little brown bats were brought to the bunker from Vermont and New York states. The bats spent the winter in the sealed bunker and were monitored throughout their three month hibernation with the infrared cameras. Bats were observed occasionally waking up and utilizing the pools of water for drinking. All of the bats preferred to roost in clusters on the vertical concrete walls at the front and back of the bunker. Only one of the bats chose to roost for a short duration of time in a hollow log that was suspended from the ceiling of the bunker.

Exterior of bunker, late MarchIn late March, state and federal biologists from New York, New Hampshire, Vermont, and Maine returned to the bunker to collect the bats and return them to their native hibernacula from which they came. Of the original 30 bats that were brought to the bunker, nine survived the winter in the bunker. While some of the bats survived the capture, transportation, and hibernation, it is clear that increasing the overwinter survival rate is essential when considering the value of further developing artificial hibernacula.

The need to remove infected bats from the wild and place them in a temporary site where they could hibernate in a cleaner environment highlights the dire situation that bats now face in the northeast. The project shows that little brown bats can survive being transported during hibernation through multiple states and that bats can survive overwintering in an artificial hibernaculum such as a military bunker. Retrofitted bunkers in northern Maine may very well serve as little brown bat hibernacula in future years or as mitigation facilities in cases of further fungal or disease outbreaks.

Steve Agius is Assistant Refuge Manager at Moosehorn and Aroostok National Wildlife Refuges.


For more photos, visit our Aroostook NWR Bat Bunker Project Flickr set