I have spent most of my life thinking bats were largely solitary creatures, apart from when they gathered in roosts in attics and, classically, church belfries. When I eventually saw fruit bats flying in vast flocks to and from their roosts in Australia, I was blown away.
Of course, I had seen images and footage before, but the sight of these huge bats winging over my head was awe-inspiring – so much so that I’d forget they were bats and find myself misnaming them as birds.
Little did I know that there’s an even more impressive bat phenomenon that occurs every year around a bridge in Austin, Texas, USA – the annual roost of the Mexican free-tailed bats.
This large family within the bat tribe contains some 110 species that are found throughout the world (apart from Antarctica), typically roosting in abandoned mines or tunnels. Free-taileds are generally quite robust, and their relatively long, narrow wings help to make them among the fastest flying bats.
Incidentally, the ‘free-tail’ part of their name refers to a special ring of cartilage that slides up or down the tail vertebrae, by muscular action, in order to stretch or retract the tail membrane. This allows the bat a degree of fine-tuning in its flight manoeuvres, enough to rival swifts and swallows, its daytime ecological equivalents.
Native to the Americas as well as the Caribbean, the Mexican free-tailed bat – also known as the Brazilian free-tailed bat – measures around 22cm and can weigh in at more than 1kg. The species wears the crown for highest flying bat in the world, reaching altitudes of 3,300m. Add to that accolade the claim that it has the fastest horizontal speed of any animal, clocked at a ground speed of just under 160kph, and it’s clear that this is some kind of super-bat.
The Mexican free-tailed bat is also regarded as one of the most abundant mammals in North America, yet its predilection for roosting in humongous numbers in localised areas makes it vulnerable to habitat destruction.
In Texas, the bats are usually summer migrants, wintering further south in Mexico. Unbelievably, up to 1.5 million individuals reside under just one bridge, the Congress Avenue Bridge in Austin, Texas. Following its renovation in 1980, this landmark was transformed into an ideal daytime roost and, in the intervening decades, the number of bats there has swelled to current levels. The incredible sight of a multitude of bats emerging from the roost at nightfall attracts more than 100,000 visitors annually, tourists and locals alike.
The bats start arriving in late March, up to peak time in August. Most of the bats are female, giving birth to their pups in June. The best time to witness the show is around sunset, when the animals start flooding out into the night, with the whole evacuation taking up to 45 minutes.
It is the largest urban bat colony in North America, and the stats are mind-boggling. Each night, the animals gobble up 4,500–13,600kg of insects. Naturally, this means tonnes of guano. During the American Civil War, this was mined for use in the manufacturing of gunpowder, and by the early 20th century guano was the state’s largest mineral export. Some is still mined for use in natural fertiliser.
So, if you want to see the state’s designated flying mammal, find a good spot, grab a taco and enjoy the bat dance of a lifetime!
Discover more amazing wildlife stories from around the world
Top image: people gather on Congress Avenue Bridge to watch a roost of bats emerging, on 10 June 2005. Credit: Getty