"There aren't many dens that can support survival over winter on this landscape," Jørgensen told me. An ideal den must be deep underground, where the earth is warm, but accessible from the surface via burrows or natural fissures. Such refuges are few and far between. "Because of that, what you get is very large aggregations of snakes at these communal dens." Picture a serpentine tangle of a thousand snakes, piled together cozily, calm and sleek in their subterranean nook, jointly awaiting the signals of spring. When surface temperatures rise to a comfortable threshold, they emerge. For a while they bask in the sunlight, crowded like bronzed tourists on the Costa del Sol. But the rattlesnakes are hungry. What's their next imperative? To get away from one another, find food, and mate. So they migrate radially—in all possible directions away from the den—like starburst embers from a Fourth of July rocket.
Jørgensen used small radio transmitters, surgically implanted, to chart this pattern, tracking the individual routes of 28 different rattlesnakes during 2004 and 2005. More recently, on a blazing summer day, he took me back to one of the den sites, in a slumping bank above the South Saskatchewan River. The slumping had opened deep underground cracks in which roughly 60 prairie rattlers had wintered. From the riverbank we turned toward the uplands and began retracing the migration route of one of his animals, an ambitious female he had labeled E.
Not far upslope were three rounded boulders, lichen-covered, with a hole beneath. Snake E had arrived here on May 8, Jørgensen said; she rested, basked, and took off again on May 27. She ascended this steep bench (we started climbing) amid the sage and crumbling gray mud, then slithered back down the slope (we plunged after her), crossed this dirt road, crossed this moist gulch full of goldenrod and skunkbrush (we thrashed through), and climbed again. Back atop the bench, we ducked between strands of barbed wire into the corner of a crop field irrigated by center pivot. The crop had been alfalfa when E came through; this year, potatoes. We politely circumvented the spud field and picked up her track on the far side, between several more center-pivot circles, blooming bright yellow and rank with canola. The midday air, hot and thick, smelled like baked fish from an oven.
Having sprinted across two pivot fields in a single day, brave lady, E had then picked up the security of a fence line, where the weeds were dense and the discs of a tiller, the blades of a swather, never touched. By late June she was making 200 yards daily, still along the fence line, amid a hospitable jumble of rocks, weeds, and rodent burrows. At that point Jørgensen and I paused in the shade of a cottonwood to rest. We had covered eight weeks of rattlesnake migration in four hours and were drenched with sweat.