Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
Gray Bat Exit Counts at Two Caves near Grand Lake and Passage/Flyway Enlargement – June 12 and 19, 2008
Southwest Region, June 12, 2008
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  Author: Richard Stark


Date(s): June 12, 2008

Location: Beaver Dam and Twin Caves, Delaware County, Oklahoma

Participants: Richard Stark and Anita Barstow, Oklahoma Ecological Services Field Office; Steve Hensley, Ozark Plateau National Wildlife Refuge; Keith Martin, Rogers State University; Bill Puckette, Contract Biologist; and Sam Ziara, Grand River Dam Authority

Executive Summary: Exit counts were conducted at two gray bat maternity caves, Beaver Dam and Twin Caves, Delaware County, Oklahoma, on June 12, 2008.  The maternity colonies are monitored to gather data on population trends and to provide information needed for an ongoing consultation with the Federal Energy Regulatory Commission (and GRDA) and the U. S. Army Corps of Engineers (Corps).  The water level in Beaver Dam Cave is affected by water levels in Grand Lake.  The Federal Energy Regulatory Commission (and GRDA) and the U. S. Army Corps of Engineers control the power and flood pools of the reservoir, respectively.  The Service anticipates take of gray bats within Beaver Dam Cave near Grand Lake due to flooding associated with the lake operation that can result in inundation of the main flyway used by bats to leave the maternity roost area and exit the cave.  Previous site survey work indicates that the flyway becomes completely inundated when lake elevations reach about 753-feet National Geodetic Vertical datum (NGVD).  On June 12, 2008, the Corps projected water levels in Grand Lake to crest at about 751.2-feet NGVD.  Significant additional rainfall was projected to occur within the reservoir’s drainage area.  The Service was concerned that the main flyway would become inundated and that take of adult females and non-volant young would occur as a result.  A possible conservation measure to reduce the amount of take had been identified by the Service during an exit count effort conducted on July 19, 2007.  The Service discovered two smaller (< 8” wide and tall) higher elevation openings that would not become inundated even when the reservoir reached the top of the flood pool.  It is highly likely, however, that the small size of these openings limited their use as a flyway for bats, especially when considering the size of the bat colony (about 12,250 bats based on the June 12, 2008 exit count).  The Service had anticipated enlarging the higher elevation passages during the fall of 2008 when gray bats would not be present because increasing the size of the passages should reduce the amount or extent of take by providing an alternative exit/entryway for the bats when the main flyway is inundated.  Due to concerns that the main flyway would become completely inundated over the next week (based on projected rainfall and discussions with the Corps regarding their ability to release water from the reservoir), Richard Stark and Bill Puckette entered the cave after the emergence of the adult females on June 12, 2008, to increase the size of the higher elevation openings.  Thousands of juvenile, non-volant young were observed on the ceiling of the roost room.  Therefore, work conducted to enlarge the passages was conducted from the entry room towards the roost room as quickly as possible.  The highest elevation passage was enlarged such that it now connects with the other high elevation opening to form a single passage.  An average-sized adult human now could move through the new passage between the entry and roost rooms.  The reservoir crested at 752.48-feet on June 17, 2008.  Richard Stark and Steve Hensley returned to Beaver Dam Cave on June 19, 2008 to see whether the gray bat colony continued to roost in the cave after creation of the additional flyway.  The entire colony appeared to be present based on the bat emergence.  The exit count at Twin Cave (on June 12) indicates that a smaller colony of about 3,400 gray bats was utilizing this cave.  Gray bats have been known to move between Beaver Dam and Twin Caves during the summer.  The Service and Rogers State University will continue to monitor these gray bat maternity colonies over the remainder of the active season to gather additional data on use of the two caves by gray bats and to assess the effects of flooding and passage modification.  Additional passage modification work also is anticipated to occur during the fall of 2008 after the colony has migrated to their winter hibernaculum.

FWS programs involved: Ecological Services and Refuges

External partners involved: U. S. Army Corps of Engineers, Federal Energy Regulatory Commission, the Grand River Dam Authority, and Rogers State University

Contact Info: Terry Whittaker, 918-382-4523, terry_whittaker@fws.gov
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