Eckert James River Bat Cave. One of the largest Mexican free-tailed bat colonies in the world is found in the hills of Mason County. Managed by the Texas Nature Conservancy, Eckert James River Bat Cave, a maternity bat cave, is home to more than 4 million Mexican free-tailed bats. Only females inhabit the cave, inside which they bear and rear their young each spring; they depart from the cave in mid-October. You can watch in the evening and morning as the entrance to the cave swarms with female bats leaving and returning from an evenings hunt to feed their pups. Stand clear of the entrance, unless you don't mind bat guano (droppings) or having thousands of female bats buzz by. The best way to glimpse this phenomenon is from a safe distance a few hundred yards away. 325/347–5970. $5. 6–9 Thur.–Sun. late May–mid-Oct. 6.