The Mexican free-tailed bat (Tadarida brasiliencis mexicana) is the most common bat found in the state. The bats emerging from nursery colonies found throughout Central Texas provide great opportunities to see millions of bats depart their daytime roosts and head out
for their nightly feeding. While many bat species roost in caves, Mexican free-tailed bats have adapted well to the urban environment, finding bridges to be an ideal environment to raise their pups.
Austin boasts the largest urban bat colonies. Bats live in the I-35 bridges at Wells Branch and Howard Lane in Pflugerville and the McNeil Bridge in Round Rock. The McNeil Bridge has approximately 1.8 million bats in residence. The Congress Avenue Bridge is a popular viewing spot with approximately 1.5 million bats. From April through September, Good Water Master Naturalists volunteer at the McNeil Bridge on Friday evenings to help educate the public about the bats.
Mexican free-tailed bat eat an amazing number of insects each night. A million bats consume 10 tons of insects each night, greatly reducing the need for pesticides. They provide a special benefit to the agricultural community by eating such insect pests such as corn earworm moths.
Wintering in Mexico, Mexican free-tailed bats begin arriving in Texas in February. And continue to arrive through April. Pups born in June stay in the roosting area while the mothers search for food each evening. The pups roost in densities of up to 500 per square foot. An amazing sense of smell and hearing allow the mother to find her pup by the scent and call. In wet years, the mothers stay close to the roosting area, but in dry years mothers go much farther to forage. The pups begin their flights in late July. August offers the best viewing as the pups have been weaned and join the parents on the nightly flights searching for insects. Male bats live apart in much smaller bachelor colonies.
The timing of the bat exodus varies throughout the summer and is affected by heat and humidity. For best viewing, plan to arrive well before sunset and wait patiently to view the beautiful spiraling exit flights. The show can last for an hour or more.
Bats are not a threat to humans, but there are some important safety issues. A bat on the ground or a bat that allows a human to approach is not a healthy bat and should not be touched because it can have rabies. It is also better to use low voices and avoid the use of bright lights or flash to avoid disturbing the emergence patterns.
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