Click. Click. Click-click. Click-click-click-click. Click-click was virtually the only sound heard, with occasional wind gusts howling through nearby canyons of a desert range, as the researchers thumbed their counters to register a bat count.
Brown's research efforts have allowed her to accurately predict when various bat species will leave their roosts to feed. This particular bat species, the California leaf-nosed bat, exits to feed when the sunlight fades and landscapes become silhouettes.
Brown had help from her husband, Robert Berry, and friends Chris Bates of Yuma and Glen Miller of Imperial on this night. An average was taken from three counts, made possible by night-vision scopes, and Brown estimated a total of 2,500 bats flew out of the mine shaft.
Brown began researching bats in 1968 when she banded bats along the Colorado River between Yuma and Parker, Ariz. to study their migratory patterns and lifespan.
Her work on the California leaf-nosed bat in the Picacho and Cargo Muchacho mountains is different.
"We have found virtually no migration with these bats. Now we are monitoring bat populations, particularly in relationship to bat gates. These gates protect bats and people," she said.
A bat gate was installed at this mine shaft by the U.S. Bureau of Land Management several years ago. The gates are typically made of thick iron and span the shaft entrance, bolted to surrounding rock. Built for permanency, the gates are barred to allow passage of bats while excluding people from the potential danger of an abandoned mine shaft.
"With this gate in place, the bat population has doubled. We are just tracking numbers. This is the biggest roost of this species in the U.S.," she said.
Prior to placement of the gate, their highest bat count was 1,200.
"In the winter these bats look for really warm mines. The temperature in this mine shaft is 80 degrees. In the summer the bats can use shallower mines and caves" she said. "Local data suggests this particular mine shaft is geothermally heated."
Brown's work in the Cargo Muchacho Mountains is focused on the effects of mining on bat populations. Before mining was started by American Girl in the Cargos, she had collected baseline data from historic mine shafts. Now that the mine has shut down, she is able to compare the baseline data to her recent research efforts. American Girl installed bat gates at open shafts as part of their reclamation effort during shutdown.
"We are monitoring their (the bats) recovery in the shafts that existed before mining began. The numbers are down at the end of mining. However, the numbers in some shafts are approaching pre-mining populations," Brown said.
According to Brown, 14 bat species exist in Imperial County. The big ears of the California leaf-nosed bat allow it acute hearing and it possesses excellent eyesight. It is a federally listed sensitive species, and Brown works closely with government agencies in protecting these bats.
Brown also is a research associate at UCLA, where she earned her doctorate in 1973. She has studied bats for 33 years and has been assisted by her husband for the past 20 years. They are from Bishop but spend eight months of the year on the road.