That's got to be a maternity colony. In the past, maternity colonies groups of female bats that congregate in summer to give birth to and rear their young were tough to locate in the wild. As a result, biologists lacked information about what bats do in summer. Most information about bats came from winter research that meant searching caves, sewers, and mine shafts when the animals hibernate. But winter searches can disturb the animals, causing them to deplete energy reserves needed to survive the cold months.
Biologists try to avoid such intrusions.
Nets strung up between trees or over streams can capture bats in summer for identification. But the effort is laborious, time-consuming, and often futile. Not until the 1970s did bat biologists begin using portable ultrasonic receivers that convert the ultrasonic sounds made by bats to locate prey and obstacles in the dark into audible sounds. Like computers, the receivers have become smaller and more accurate. What was once silent guesswork has been transformed into identifiable chirps, burrs, and chatters that scientists can pin to individual species.