A huge project covering 31 states and 10 Canadian provinces is focused on  better understanding the ecological role that bats play – and the threats they are facing from climate change, loss of habitat and the development of wind  energy.

The North American Bat Monitoring Program uses acoustic surveys – to detect high-pitched frequencies emitted by the flying mammals – as they  navigate in the dark.

The monitoring program has been spurred by a disease called white-nose syndrome.

It has killed millions of bats – and is spreading.

There are some 150 species of bats in North America – 47 of them in the United States. Some migrate more than 800 kilometres – and others hibernate in caves or abandoned mines.