ird-tracking project on Texas' Padre Island in the mid-'80s, Tennant and Vose were both intent on chasing the falcons in his somewhat ramshackle Cessna Skyhawk before satellites and computers made their airborne telemetry irrelevant.
"We wanted to see what they saw, feel what they feel," Tennant said.
That was not always pleasant. Peregrine falcons had recovered from the DDT damage and bounced back in healthy numbers, but their environmental challenges are in some ways worsening.
From the air, Tennant and Vose saw the devastation caused by Mexican petrochemical dump sites and the ravages of paraquat defoliant in Guatemala, which apparently killed one of the falcons they followed. They also saw the ravages brought to the high plains by cattlemen.
"These early cattlemen didn't bring ruin to the plains by killing the buffalo and fencing in the Comanche," he writes in the book. "Unknowingly, they attacked the grassland itself, at its roots, and grass lives by its roots.... (The cattle) walked the prairie to death."
This summer Tennant discovered that Umiat, the town on Alaska's North Slope from which he tracked the Arctic falcons to their nesting sites along the Colville River, is now a toxic waste site: Effluent from oil drilling was dumped there through the '50s.
Falcons and other wildlife are "hit with the same things that we're hit by: the PCBs, all the agricultural spray insecticides. There's a much greater environmental assault overall than there was when the peregrines went down (in numbers)," Tennant said.
But Tennant said the falcons' peripatetic moving about makes it difficult to draw conclusions.
"They move so much that they're not so much of an environmental thermometer as everyone thought a canary in a coal mine. They go such distances; they can eat one poisoned seagull and fly a thousand miles and die, then you can't pinpoint the source," he said.
Not all the adventures during their falcon trips were in the air. Tennant and Vose "borrowed" the radio transmitter and scanner from the U.S. Army, had frequent mechanical mishaps with Vose's old plane, and were held at gunpoint by Mexican drug smugglers when they landed on one of their rural airfields.
Both men live in West Texas now, where Vose, still a flying instructor at 82, operates a commercial flying service from Alpine, Texas, and uses radio telemetry to track bears and quail.
Tennant, 61 and an avid bicyclist, has written several award-winning books on wildlife and nature. When not lecturing on nature, he lives in Marathon, Texas, just north of Big Bend National Park.
His falcon tales have already captured the attention of the reading public, putting On the Wing onto best-seller lists. It may even reach the silver screen. The story has been optioned by Robert Redford to be made into a film, Aloft, for National Geographic Feature Films. Redford plans to play Vose.
Editing by Cynthia Osterman