So when will the National Audubon Society learn whether its Sabal Palm sanctuary winds up south of the new border? “I couldn’t tell you a specific date,” Mr. Knocke said. “But there should be no uncertainty about how quickly we want to move.”

Put yourself, then, in the dusty shoes of Jimmy Paz, 66, the weathered manager of Sabal Palm. At the moment he is sitting at a picnic bench outside the modest visitors center, trying to speak above some chattering chachalacas feeding on grapefruit rinds. Now and then he interrupts himself to point out the iridescent brilliance of a green jay, or to ask passing birders where they are from.

Montana, a few say. California, say others.

Mr. Paz, a native of not just Brownsville but “beautiful Brownsville,” knows the area and its rhythms. He says the Fence would create a twilight zone out of a swath of distinctive American soil, disrupt and damage wildlife and have the opposite of the intended effect: it will be the birders and other tourists  not the illegal immigrants  who stop coming. It may also put him out of a job.

“It would be like putting a fence around Central Park,” he said.

Mr. Paz remembers cycling as a boy to the “palm jungle” along the Rio to re-enact scenes from the Tarzan movies he had just seen at the Queen Theatre in downtown beautiful Brownsville. After a decade in the Army, he returned to hold a series of jobs, including police officer and windshield repairman, while the Audubon Society acquired parcels of that jungle to create a sanctuary to be called Sabal Palm, after the stocky palm trees of the Rio Grande valley.

Ten years ago he became manager of the very property where he once imitated Johnny Weissmuller  property that sits roughly between a bio-diverse preserve owned by the Nature Conservancy of Texas and a swath of land restored by the United States Fish and Wildlife Service. Combined, Eden.

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Mr. Paz has come to know those who frequent this sanctuary: the buff-bellied hummingbirds, the long-billed thrashers, the ever-prowling Border Patrol agents, the river-wet visitors from Mexico, passing through. Driving the grounds in his pickup truck, he points to a telltale inflatable tube, discarded at river’s edge.