in the Austin, Texas backyard

This week we have experienced the great Texas heat wave up close and personal. They are working on a record 42 straight days of > 100 F temps, which make it really hard to want to go outdoors. But we did venture out one night, when the already excessive air temperature was added to by the re-radiation from the concrete and steel of the downtown buildings. Why did we subject ourselves to this? To see the emergence of 1.5 million Mexican free-tailed bats at sunset from the Congress Street bridge. This daily occurence is quite famous, and draws hundreds of people each night that line up on and below the bridge to watch.

Minute cracks in the underside of the bridge are the perfect size for these bats to climb (or fly) in and create their communal roosts. They arrive here (from the south) in March, and breed and raise young, migrating south again in September. I assume the moisture along the cracks is due to bat excreta.

We walked along the river while waiting for the bats to emerge on their foraging flight, and I found a dead one on the sidewalk. You can clearly see the free tail; the tail of other bat species lies within the membrane that runs between their hind legs.

At last, just as predicted by the local news and hoteliers, the bats began to emerge from the far end of the bridge across the river, and then in successive waves from each of the slots along the underside of the bridge. They flew in a straight line down-river, with the entire emergence probably taking about 15-20 minutes.

According to Wikipedia, this mob consumes 10,000 to 30,000 pounds of insects EACH night.
After this excitement, we had to retreat to a hotel and drink about a gallon of water to rehydrate. I can't imagine what the air quality inside those roosts is like with a million and a half hot little bats heating up the enclosed space and producing great quantities of ammonia as they digest their evening meals.