Austin’s famous bat colony (Congress Avenue Bridge at the Colorado River, 512/416-5700) is a major source of civic pride (and guano). Each summer, more than a million Mexican free-tailed bats pour out nightly from beneath the downtown Congress Avenue Bridge in search of food, and scores of Austinites and visitors from across the world eagerly witness the spectacle.

Part of the draw is the anticipation, but the greatest appeal is seeing an undulating mass of chirping mammals flow from their concrete catacomb into the deep blue sky of a pristine Austin evening.

But what exactly are these creatures?

Mexican free-tailed bats are considered one of the smaller species of bats, with a wingspan of 11–13 inches. They’re named free-tails because the lower half of their tail is free of a body-attached membrane. These little critters are typically a dark to light brown color.

Found mostly in the western United States and Mexico, it’s estimated that 100 million Mexican free-tailed bats arrive in Central Texas annually to raise their young. Nursing females require large quantities of high-fat insects, so they tend to feast upon egg-laden moths, including bollworm moths, cutworm moths, and other agricultural pests that migrate north from Mexico.

Free-tails usually emerge about 15 minutes after sunset. They leave the bridge (or, in most other locales, a scary cave) at speeds of up to 35 miles per hour, and they can increase in velocity up to 60 miles per hour. A single bat pup is usually born between June and July, and the little ones take their first flight at nearly five weeks old.