Bat research near Aspen
CARBONDALE, Colorado ” Tucked into a crack on a shoulder of Mount Sopris lives a bat colony so unique that experts from around the region have come to study it.
Somewhere between 500 and 750 Townsend’s Big-Eared Bats live in an old lead mine that intersected a natural vapor cave. The warm underground cavern creates an ideal habitat that draws Townsend’s bats in much greater numbers than usually found in one place. They don’t live in the Sopris bat cave year-round. Instead, it’s the ultimate nursery for the adult females, which return there each spring to give birth and raise their young.
“We have found only 15 maternity roosts statewide,” said Kirk Navo, a bat biologist with the Colorado Division of Wildlife. Most of them attract 15, 30, maybe 50 bats. Only two maternity sites are used by hundreds of bats. The Sopris bat cave is the largest maternity roost found west of the Continental Divide.
“It’s a very unique and significant site,” Navo said of the mine between Carbondale and Redstone. Officials don’t want its exact location disclosed because they want to discourage visits.
The geothermal feature makes it attractive to the furry flying creatures. Temperatures in some of the adits, or horizontal cuts of the mine, hover around 75 degrees, even in the winter. That is perfect habitat because all the bat pups’ energy can be channeled into growth rather than simply survival, Navo explained. The adults give birth to one baby, called a pup, per year.
In addition, the adult females have an abundant food supply nearby. Moths are thick in a meadow by the Crystal River.
U.S. Forest Service biologist Phil Nyland of the Aspen-Sopris Ranger District has been among the experts who have studied the Sopris bat colony over the past four years. He and his colleagues have waited outside the cave entrance shortly after dusk and filmed the bats emerging for feeding. It’s a site he won’t forget.
“You see bats coming out of these portholes like bees out of a hive,” Nyland said. “It’s not like a cloud. It’s like a stream.”
The stream flows for up to an hour, he said. The bats re-enter the cave at about 3 a.m. In between those times are frequent flights in and out by some of the bats.
Adult Townsend’s Big-Eared Bats have a body length of only about 4 inches, according to state wildlife division material, but their ears stick up one and one-half inches ” thus their name. They are considered excellent fliers due to a wingspan of about 10 inches.
The same qualities that make the Sopris site so good as a maternity roost make it lousy for hibernation, Navo said. The bats need temperatures in the low 40s to mid-30s. Since the old mine doesn’t fit the bill, the bats depart each year near the end of September. They return each spring, some as early as March.
“I don’t know what triggers them to come,” said Navo, “nor do I know where they’re coming from.”
Research indicates the bats remain in Colorado rather than migrate a long distance away to hibernate. Navo said they probably travel 20 to 30 miles to find caves and other openings that get cold enough for hibernation.
Researchers were alerted to the maternity colony by local miner Robert Congdon, who has poked around the old mine on the side of Sopris for nearly 30 years. He holds the mineral rights; the Forest Service holds the surface rights at the site.
The mine produced lead for smelters in Aspen, indicating it was active around 1890, Congdon said. It was also mined for zinc and copper. The mine produced iron oxide for paint pigment, possibly into the 1940s, he said.
Congdon said he initially noted only a small number of bats some number of years after he first started visiting the mine. The numbers increased as time went on, and he suspected something special was occurring. He guided Navo and other researchers on their first visit to the site.
“They were giggling like little kids when they saw all the bats coming out,” Congdon recalled.
Navo has worked 18 years in a program in which he has examined abandoned mines to see if they are bat habitat. The goal is to make sure the mines that are being used aren’t sealed in human safety programs.
Navo labeled the Sopris mine a “deluxe” site. “It’s so good to find a site that has such a big number,” he said, noting that the Townsend’s Big-Eared Bat is considered a species of “special concern” in Colorado.
But the Sopris site has problems. The mountain is sloughing rock and dirt at the main entrance. Officials are concerned that the material could eventually trap the bats if nothing is done. Paul Krabacher of the inactive mine program at the Colorado Department of Natural Resources, designed a manmade entrance intended to provide permanent access for the bats.
A culvert 3 feet in diameter will be placed in the mine, and it will jut out past the area where material continues to fall. Townsend’s don’t like tight spots, so the team came up with a plan to make a new entrance that is more enticing than a culvert opening. The culvert will attach to a cupola, or big box, made from metal bars. The box will be either 6-by-6 feet or 8-by-8 feet, Navo said. The metal bars are spaced 6 inches apart.
Cupolas have been used effectively at other mines used by bats, but usually it is placed over a vertical shaft, Navo said. He is confident this design will work, but it will be monitored next spring when the bats return, and adjustments will be made, if necessary.
A contractor has been hired for the $51,000 project. The wildlife division and Forest Service are contributing $6,500 each, with the Colorado Division of Reclamation Mining Safety picking up the bulk of the cost. The project will likely take two weeks.
“We’re itching to get going. All of us are,” Nyland said.
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