Just before the last faint light of dusk fades, two or three bats swoop out of the deep tunnel.
They’re the scouts. They flit cautiously around the immediate vicinity, scanning for hawks, owls or other predators.
Seeing no danger, they hover momentarily at the tunnel opening and signal the others inside.
Then wave after wave of small winged mammals emerge, fanning out by the thousands for a night of hunting. Many will head for the rural areas of the East Valley to feast on insects in crop fields.
They’re not departing from somewhere beyond the urban area’s outskirts, where a large bat colony normally would find a suitable haven. Their home base is next to a city neighborhood, close to homes, office buildings, apartment complexes, schools and busy streets.
The Mexican free-tailed and western pipistrelle bats have found a hospitable habitat inside a large tunnel that’s part of a flood control channel near Camelback Mountain in Phoenix.
Tucked into the crevices of the tunnel’s high concrete ceiling, the bats can stay sheltered from the sun and out of reach of intruders. Plus, it’s conveniently located next to a big water supply, a stretch of the Salt River Project’s Arizona Canal.
The Flood Control District of Maricopa County and the Arizona Game and Fish Department have designated it a protected wildlife area. Typically a dozen or more people gather each evening outside the fence enclosing the tunnel opening to watch the bat brigades take flight.
This is breeding season for the colony. The estimated 3,000 bats there could soon become more than 5,000, said Game and Fish biologist Nancy Renison.
More than offering an unusual spectacle of the animal kingdom in the middle of a large urban area, the bat colony is serving a valuable environmental control function.
Often flying as many as 40 miles from the roost site each night — farm lands in the Chandler and Gilbert area seem to be a favorite destination — the bats are eating large quantities of mosquitoes, moths and other pesky insects along their routes, Renison said.
The bugs that devour plants and irritate people would be noticeably more abundant if not for the bats’ voracious feeding habits, said Theresa Pinto, environmental planner for the county flood control district.
"They eat bugs by the millions," Renison said, some up to 600 mosquitoes in an hour.
Urban refuges also are helping bats, which are losing habitat in Arizona. Even in rural and remote areas, caves and old mines long used by bats are being closed for public safety reasons, Pinto said.
The Game and Fish Department helped form the Arizona Bat Conservation Partnership last year to enlist government wildlife and land management agencies and environmental groups in efforts to protect bat habitats.
Fortunately, the cause is being aided by resourceful bats like those that have made a home in an urban flood control tunnel.
The path to the tunnel is on the north side of Arizona Canal on 40th Street just north of Camelback Road. Head west about 200 yards then north to a paved road and follow it about 50 yards to the top of the flood control opening. Parking in the area is limited.