On a Friday evening in the middle of August there isn't much traffic along Franklin Boulevard, an old agricultural road that now cuts through 20 miles of Sacramento, Calif., suburbs. We're looking for birders along its half-mile length, but we don't see anybody, or even a parked car.
When a farmer drives his pickup out of a field and stops to lock a gate, I walk over and ask whether he's seen any hawks or bats in the neighborhood. He sure has, he says, because his family has farmed nearly 400 acres of this land for six generations. They're Mexican free-tailed bats, he says, and there are 30,000 to 40,000 of them roosting under the bridge right now. The hawks gather when it's time for the bats to emerge, and we'll hear them, he assures me.
As if on cue, a Swainson's hawk screams.
By the time we park and get across the road, four or five Swainson's have begun circling low over the riparian oak woods. We stand at the bridge's railing and peer at the floodplain below. A few bats shoot out. Soon a stream of the tiny mammals pours down from under the bridge, and the stream becomes a torrent. Bats swirl into a great boil before making a dash for the nearest oaks, where they rise into the canopy and form ribbons that wind westward into the orange sky.
That's what the hawks were waiting for. I spot a bird positioning itself above the nearest ribbon of bats, and then it plunges through, talons outstretched. A bat, still flapping, gets caught in a talon. The hawk quickly dispatches the bat and veers back for another pass. Now there's action all around, with thousands of bats unspooling into four great ribbons to fly through this gauntlet of Swainson's hawks.
It isn't only hawks. Two peregrine falcons have joined the hunt. Within minutes all the raptors have bulging crops.
Turning my binoculars toward the distant lead bats, now far out over the valley, I wonder how much longer this remarkable hunt can last. Despite the recession and the efforts of local conservation groups, Sacramento continues to sprawl, gobbling up farmland, grassland and woodland to spit out legions of McMansions, retirement communities and strip malls. With each advance of the developers' bulldozers, Swainson's hawk habitat is destroyed.
But a more immediate threat may come from the wind. Famous for their long-distance migration of more than 10,000 miles, Swainson's hawks follow the wind as far south as the Argentine pampas. Energy companies also follow the wind. Seven wind farms have been built in the Isthmus of Tehuantepec, Mexico, near a stretch of the flyway used by the vast majority of Swainson's hawks. Thirteen more are in development. Erecting thousands of wind turbines along a major migration corridor would seemingly fail a fundamental requirement for bird-safe wind energy: correct siting. A World Bank document about one of the Tehuantepec wind farms states "avian impacts are not expected to be significant," but a case study of another wind farm admits "concern about the potential cumulative impacts of the many additional wind farms planned in the same general area."
Of course, the dangers posed by wind energy don't lie just south of the border. California has its own wind farms, and some are notorious for their raptor mortality. Several wind farmscomprising more than 700 turbineshave already been constructed in the Montezuma Hills of Solano County, an important part of the Swainson's hawk's historic breeding range.
Reviewing a draft environmental impact report for the latest proposed wind farm there, Friends of the Swainson's Hawk, a wildlife advocacy group, warns, "Over time, the project could have a significant impact on the range of the Swainson's hawk in California through higher than natural mortality rates and loss of foraging habitat used by nesting Swainson's hawks to feed their young." The project would be relatively small, at just 50 turbines on about 3,000 acres, but it can only ratchet up the pressure on this threatened, distinctive population of Swainson's hawk. Some unknown number of birds will be killed each year; more habitat will be lost.
After 25 minutes, the bat exodus is coming to an end. We walk down the bridge, following the last bats across the Mokelumne, and almost run into a peregrine flying toward us. Startled, the bird banks and heads upriver into the night.
John Berry is a contributor to Writers on the Range, a service of High Country News (hcn.org). A Californian, he now works and usually birds in New Jersey.