The Congress Avenue Colony
The colony consists of 500,000 to 1.5 million Brazilian free-tailed bats, roosting and rearing young in 3/4 to 1 1/2-inch-wide by 16-inch deep crevices between concrete beams.
The massive concrete beams serve as thermal heat sinks, providing ideal temperature ranges for rearing young.
Emergence columns from the bridge are visible for more than a mile, sometimes climbing thousands of feet above ground. Emergences include from one to five columns.
Mothers normally remain at a single location while rearing young. But males and females not rearing young move among several bridge roosts in Austin, and large numbers sometimes move in a single night between Austin bridges and major roosts in Texas caves.
Some travel up to 100 miles or more in a single night to find the four to six tons of insects required to sustain the colony. Most feed on agricultural and yard pests greatly reducing needs for pesticides that threaten environmental and human health.
With the arrival of a major cold front in October or November most members of this colony migrate south into Mexico, where they remain active till their spring return in March and April.
These bats do not hibernate and consequently are not susceptible to White-Nose Syndrome.
The most recent emergences based on Doppler weather radar, courtesy of Austin Bat Refuge.
Economic and Ecological Contributions
Free-tailed bats save farmers millions of dollars annually in avoided pesticide use.
They intercept billions of migrating moth pests each spring, preventing egg-laying on crops. One bat can easily prevent more than 20,000 eggs from being laid on crops in a single night.
Unique microbes from free-tailed bat guano ecosystems in caves can be of great biotechnological value in detoxifying human wastes, even aiding in gasohol production.
Prior to the discovery of oil, guano fertilizer, extracted from these bats’ caves, was the biggest mineral export of Texas.
Free-tailed bats of Central Texas consume close to 200 tons of insects on an average summer night, essential to keeping ecosystems healthy.
Protecting Congress Ave Bridge Bats
(Advice for city managers)
Bat use of the Congress Avenue Bridge appears to have declined over the past 20 years. To improve viewing opportunities, we recommend:
Exclusion of bands from the bridge during "Bat Fest." Loud bands are delaying emergences on the nights when the largest crowds are present to observe the bats.
Trimming vegetation to maximum of 4 feet tall beneath the south end of the bridge. Currently untrimmed vegetation is preventing bat emergence over the Statesman Viewing Area, greatly reducing visitor enjoyment.
Removing hanging electrical wire and light bulbs from beneath west side of Congress Avenue Bridge. Collisions kill young bats.
Exclusion of howitzer firing during Auditorium Shores 4th of July fireworks and consider moving the event farther from Congress Ave Bridge to avoid harm to bat hearing.
“Living in harmony with nature is essential to human well-being.”
Merlin Tuttle | MTBC Founder
About Brazilian Free-tailed Bats
- They form the world’s largest colonies of non-human mammals, some including 10 - 20 million individuals.
- A single colony can catch more than 100 tons of insects in just one night.
- Their favorite foods include costly crop pests such as corn earworm moths, armyworm moths, and cucumber beetles. They also eat mosquitoes.
- Free-tailed bats save Texas farmers an estimated 1.4 billion dollars each summer.
- They rely on long, narrow wings and other adaptations to achieve flight speeds of nearly 100 miles per hour, aided by tailwinds.
- They fly up to 10,000 feet above ground.
- The Congress Ave. Bridge nursery colony begins to form in March and begins leaving for Mexico by late October.
- This species ranges from coast to coast in the southern U.S. and from Mexico south to Argentina. It also lives on Caribbean Islands.
- The largest colonies live in caves, but can also use abandoned mines, bridges, cliff-face crevices, buildings, custom bat houses and hollow trees.
- Cave colonies can survive ammonia concentrations immediately lethal to humans (Ammonia produced by dermestid beetle larva feeding on guano).
- Most free-tailed bats from Texas migrate south into Mexico in October/November and remain active year-round, returning to Central Texas in March/April.
- Groups of hundreds of thousands may travel together, apparently sharing information on feeding and roosting locations.
Courtship and Rearing Young
- Males sing to attract mates.
- Mothers produce only one pup annually, typically in early June in Texas.
- Babies are born hairless, about a quarter their mother’s size, the equivalent of a human mother bearing a 30-pound baby.
- The young grow rapidly and learn to fly in 4 to 5 weeks.
- Four hundred pups can roost in a single square foot on cave walls, yet mothers find and nurse their own several times daily based on voice and scent.
- Mothers sometimes adopt orphans.
- Baby bat photos!
Bats are America's most rapidly declining and threatened warm-blooded animals. Alarming losses of free-tailed bats have been reported though their population status is inadequately monitored. Even the Congress Ave. Bridge bats appear to be in decline.
03. Conservation needs
Public education regarding the needs and values of these and other bat species is urgently needed as is research to better document their many benefits to humans.