Mexican free-tailed bats emerge from North Campbell Ave. bridge over the Rillito River July 12, 2008. The bats take flight just after sunset to feast on insects.

The old mine now hosts a colony of more than 100,000 Mexican free-tailed bats that arrive each year in late spring and stay through the summer. “They just showed up a week ago,” Frederick said.

The Mexican free-tailed bat maternity colony under the I-80 Yolo Causeway is growing daily as they return from wherever they ...