A wave of frigid air triggered “hypothermic shock” in a colony of bats clinging to their roost beneath a bridge in Houston, a sprawling city in Texas in the United States.

Nearly frozen, the bats began losing their grip and falling to the pavement from 9m up, the Houston Humane Society reported on its Facebook page.

The Mexican free-tailed bats “are tiny, and have minimal body fat, so they are not able to live long when laying exposed on the ground in freezing temperatures”, the Humane Society explained.

As the cold wave intensified last Thursday, the group launched a bat rescue, gathering up 929 bats from the ground under the Waugh Bridge, and providing them with heat and nutrition.

Volunteers joined in the effort and more than 1,500 bats were taken in by the weekend, kept warm at the shelter or in the attic belonging to society wildlife director Mary Warwick.

Most only needed warmth and water, but the most affected were placed in incubators and fed intravenously.

“Amazingly, most of the bats have survived,” the society said.

With temperatures climbing to 22°C yesterday, the society released “close to 700” of the bats back to the bridge, leaving them free to swoop through the skies for insects.

Bat watching is a popular pastime in Texas, and a number of bridges have huge colonies. The Waugh Bridge in Houston, Congress Bridge in Austin, and Camden Street Bridge in San Antonio all draw visitors at dusk, when the bats leave en masse for nightly feeding forays.