Bats of Old Tunnel Wildlife Management Area

Local residents report that shortly after the removal of the railroad tracks in 1942, bats moved into the tunnel. In July and August there are over three million Mexican Free-tailed bats (Tadarida brasiliensis) and 1,000-3,000 Cave Myotis (Myotis velifer) in Old Tunnel. The Mexican Free-tailed bat is a medium-sized bat, weighing about 12-15 grams (12 paperclips). They are named for the tail that hangs free from the uropatagium (skin between hind legs and tail). The title "Mexican" is added to their name because they migrate to Mexico in the winter months. Old Tunnel bats usually leave the tunnel in late October, and fly to Mexico. During this migration, they can fly at altitudes of 10,000 feet and speeds of 60 mph.

A Year in the Life of The Old Tunnel Bats: While in Mexico, these bats mate during February and March when the females are most fertile. Bats returning to Texas in late March or early April include pregnant females who give birth to one single pup (baby bat) during June. The pups are not born in the tunnel, but in other nearby caves or under bridges where the temperature and humidity levels are more stable. The tunnel is only 920 feet long and is open at both ends, and therefore, the temperature and humidity vary greatly. The Old Tunnel is considered a pseudo-maternal colony because there are both pregnant and lactating females in the tunnel, but there are no pups. Pups usually fly within four to six weeks after birth and are considered juveniles when they travel to the tunnel with their mothers. Bat populations in the Old Tunnel increase substantially when the females return from maternity colonies along with the juveniles in late July and early August. Joining the Old Tunnel population in September and October, are bat migrating south from further north in the United States. This process of "staging" occurs when bats group together before flying to Mexico for the winter.

What Happens When the Sun Goes Down: On summer nights, the Old Tunnel bats usually emerge within an hour before or after sunset. Most bats exit through the south end of the tunnel, spiraling in a counter-clockwise direction in order to gain altitude over nearby trees (click here to view video images of bat emergences). Red-tailed hawks are sometimes seen feeding on the bats as they emerge, however these and other predators do not impact the bat population. The large serpentine column travels southeast towards the Guadalupe River. Bats also exit from the north end of the tunnel, and fly either north towards the Pedernales River, or south over the Old Tunnels' observation deck. After getting water from local water supplies, the colony disperses to feed in smaller groups. Eighty percent of the Free-tail's diet includes agricultural pests such as the cutworm, cornborer and webworm moths. Each can eat its weight in insects nightly, and the whole colony may devour over 25 tons of moths per night. The bats return to the tunnel between midnight and daybreak, having traveled an average 25 to 30 miles to forage.

Good Ole' Bat Guano

Huge deposits of droppings in the roosts of Mexican Free-tails have earned them the common name, "Guano bat". Bat guano (droppings) from insectivorous bats was an important source of fertilizer, and is still sold as an expensive, organic fertilizer. Before the oil boom, guano was Texas' number one mineral export. Many have tried to harvest guano from Old Tunnel. However, natural springs within the tunnel made the operation too difficult. Many Free-tailed colonies consist of at least one-million bats, and the resulting concentration of ammonia is extremely high. In fact, many people can identify these bats by the distinctive odor.