OMAHA, Neb. (AP) — Spectators have gathered to watch tens of thousands of birds swirl like black leaves in the sky to roost in a row of ash trees in Omaha.

Each evening in late summer, numerous purple martins and a lesser number of grackles and starlings swirl around the trees located along a street at the Nebraska Medical Center to eventually settle for the night. Several dozens of people, including the Omaha Youth Birding Group, stood nearby to watch the swarm of birds on Saturday.

Tisha Johnson described the birds’ chatter like a rushing waterfall, calling the sound like “nothing else.”

An Omaha birder who monitors the roost estimated that 20,000 to 30,000 martins were at the roost Saturday night.