Much to my delight, I am also adept at finding feathers. I proudly present my discoveries to Booms. He glances at them and says, kindly, "Cool! Ptarmigan!" Another one. "That's a really nice ptarmigan feather." How about this one? "Yeah, that's still ptarmigan."

Clearly, gyrfalcons enjoy gorging on ptarmigan. Depending on the locale, ptarmigan make up to 95 percent of their diet. The perches are littered with clenched feet, denuded skulls, pieces of wing, esophagi (I think), other unidentifiable bits and pieces, and, of course, feathers. After my eighth or ninth attempt, Booms pulls out two gyrfalcon feathers he has found. They look identical to me, but he turns them gently in his fingers and points out subtle differences. One of them is a touch longer, a softer gray, and has some faint speckling near its edge, but the curvature of the feather shaft, or rachis, is the same on each. Booms says the larger one is from a female; the other, her mate. Considering their size and shape, they are likely both the seventh primary, or flight feather. Now, the feathers could not appear more distinct. I'm reminded how fun it is to watch a skilled biologist interpret subtle signs in the field; and Booms is still young.

One morning, we spot a male rough-legged hawk sitting on the small hill adjacent the Horseshoe. Later, we see it doing its courtship display, known as a sky dance. Every day afterwards it is there, swooping and calling, or perched on the hill. Booms wonders why the gyrfalcon pair doesn't seem to care about this trespasser so close to their nest. He worries that their chicks -- they have two this year -- might have died. But soon he hears the pair bringing back food, and he suggests that we pay a visit to see how the chicks are getting along. Although the gesture is couched as a favor to me -- most of my glimpses of gyrfalcons have left something to be desired -- I can tell that he is pleased to have an excuse to visit what he calls "the kids."

The nest in the Horseshoe is midway down the caldera wall, maybe 20 or 30 feet from the top. The basalt is crumbly and sharp and the fecal smell strong as we rappel in. On a ledge, a rather sodden assemblage of sticks and branches spills off a vegetative mat -- an old rough-legged hawk's nest. The two species sometimes trade nesting sites, a source of antipathy between them. Wedged against the rock wall are two chicks. "They're about 34 days old," Booms says. His precision here is hard-won: He had to watch (or fast-forward through) almost 2,700 hours of video from a complete nesting cycle to learn when, exactly, a chick's feathers will poke out of the tufts of down to the extent that they do now.

I secure the chick's wings and then grab its feet. This is more for its safety than my own. Falcons, and especially chicks, don't have the terrifying grips of eagles or hawks. They dispatch their prey with blows: a hard thwack to the back of the head, and then a bite to the neck to sever the spinal cord. But if I don't hold its feet, the flailing chick might put out its own eyes. I heft it. It is surprisingly light and soft. It snaps at me, but doesn't struggle. The force of its glare is almost palpable. Booms points to its bulging crop. If the carnage around the nest is any indication, both chicks are stuffed full of ptarmigan.

"I love this system," Booms said earlier. "There aren't many components, so they're easy to follow and account for." The field of ecology prefers webs of interactions such as these, with predators (the gyrfalcon), prey (the willow ptarmigan), competitors (the rough-legged hawk), and direct or indirect effects (the looming specter of climate change), all clearly defined. As long as a necessary role is played, it doesn't matter who plays it. Such webs are abstract, though; ecology doesn't do so well with specific individuals. I think of this as Booms perches on the edge of the nest and looks at the chicks with obvious affection, which they don't return. They kak and hiss and push up against the cliff. They fear for their own, individual lives. They do not believe in ecology. Neither does their mother, who flies behind us, calling. Booms says we should leave so we don't upset her too much. Maybe 10 or 20 percent of chicks survive their first year. We drop the rest of the way to the ground and walk back to camp. The chicks watch us go.

On my last morning, after Booms has left for Fairbanks and I await the floatplane that will take me to Bethel, I see the rough-legged hawk again near the Horseshoe. Suddenly, the female gyrfalcon shoots out of the crater and sweeps down on the hawk, which gives a startled squawk and flails off its perch. The female swings around, kakking wildly, and the hawk dodges out of the way. Then I see, rising from the ground, the male gyrfalcon. The hawk must see it, too, because its flapping becomes more urgent, and it zigs and zags, trying frantically to get any sort of lift. But the falcon's pace is incredible. It climbs straight up, so fast that the hawk seems still. I jump up and down, screaming, "Go go go go go go!" (I don't realize I'm doing this until I'm teased about it later.) Just before the gyrfalcon reaches it, the hawk splays its talons. But the gyrfalcon doesn't stop, and the two birds smash together, lock talons and corkscrew to the earth. Just before they hit, they release their holds. Then the hawk flaps off over the hill, and the gyrfalcon returns to the crater, and a small flock of scoters natter among themselves on the lake, and an Arctic tern does circuits about the shore, and everything is as it was before, as it will be tomorrow, when no one is here to watch.