Austin is home to the largest urban bat colony in North America. Approximately 1.5 million bats have made the Congress Avenue Bridge their home. The bats settled here after a renovation of the bridge in 1980 created crevices that were perfect for bats to take refuge in against predators. Every spring the Mexican free tailed bats migrate north to the bridge to give birth and raise their offspring. Come summer time, the bats can be seen each evening at dusk as they emerge for their nightly excursions to rid the city of pesky insects before heading back to central Mexico in the fall.