If you've ever ridden you bike along the Rillito bike path and gone under the bridge at Campbell and River, you might notice an acrid smell - that's guano. There's a colony of bats that live under the bridge and take flight right at dusk during the summertime. It's a beautiful natural phenomenon, and fun to bring kids to see.
It might seem like you're waiting a long time, or maybe they won't leave, or maybe you've missed it - you haven't, be patient.

A few bats will fly out first as the colony gets ready to leave the bridge. All of a sudden they will take off, streaming out from underneath and form a cloud. Their chirping is audible as they take off to go feed on insects.

Most likely, these are Mexican free-tail bats that have migrated here in April from Mexico, and will return to Mexico in October. They are protected by Arizona law and best viewed from a distance.

Catch their flight at dusk any evening during the summer, likely through September.