"Come on, Heather," you say, with that cold in your voice that makes my stomach clench every time. "Turn that damn thing off and let's go."

I type ". . . We'll be in the field with no Internet access for about six weeks. After that, watch this space for updates on the Green-Tailed Hummingbird!" into Facebook, hit "Share," shut things down fast, turn to look at you.

You're still the man I married. Still tall. Still blue-eyed. Standing there by the door with a worn-out duffel bag slung over your shoulder and a hole in the collar of your shirt.

"I have to update, Dave. The sponsors and the public . . ."

"Yeah." Not as cold now. "I just don't have time for that stuff. Let's go."

Your smile is the same as when we met.

You used to not hate social networking sites, or promoting our projects, or TV, or traffic, or whole chunks of the modern world, the way you do now. You used to be able to laugh at it.

I pick up my bags and follow you out the door.

You pull onto the 10 and head east into a pink and searing white-hot April desert sunrise. Something rattles in the truck's dashboard. After merging with the long-haul trucks and speeding SUVs, you reach over, put your hand on my knee. "Sorry, hon. Was I an asshole?"

It's not that you're being an asshole. It's . . .

"I'm kind of stressed that we'll get to Sombra Canyon and never see a bird. And the Conservancy wants their grant back and the Geographic deal falls through and we can't pay the mortgage and have to live in the truck. . . ." You're laughing as you say this.

But it's possible that we won't see a Green-Tailed Hummingbird.

Prosaic common name for a living jewel, Archilochus viridiens, a few ounces of iridescent black with streaming absinthe-green tail plumes on the male, green-shaded tail feathers on the female. For at least the last fifteen years, it hasn't been seen outside Sombra Canyon, in the southeastern California back country. It is rarely seen even there, and it is thought to be on extinction's razor edge.

That you might not see one, that you might fail to successfully mist-net and band several and do all the scholarship you've made your name at, bothers you, I know, more than the idea of us living out of the truck.

"We'll find them," I say.

You squeeze my knee, turn on the CD player, recoil from the first disc that comes up, something of mine, and hit the changer till you get an old Electric Skychurch disc.

Deus. Deus.

We drive on into the sun.

We met when you scuffled in your size-eleven hiking boots across a stretch of white desert sand I was cataloging for tracks. You were looking up, through binoculars, not down. I threw a notebook that hit your shoulder. You lowered the bins, took off your sunglasses, and blinked at me: blue eyes. It wasn't love at first sight.

We were married eight months later.

That was three years ago.

You swerve around a semi and say, "Did I remember to send the new grant paperwork?"

"No."

"Fuck—"

"I did."

"Oh." Ruefully. "Thanks, hon."

But you didn't exactly . . . forget. Did you? It's been this way for months. Longer. The thing with the computer. "Forgetting" to log your grad students' grades in the online system, until the department secretary starts calling our home number four or five times a day to beg. Either angry at everything that's not wilderness and wildlife, or just . . . not seeing it, somehow, like a bird can't see the mist net about to catch its wings.

I'm afraid I'm starting to be one of the things that you don't see.

We reach Sombra Canyon by a four-wheel-drive road choked with sand, rock piles, and creosote bush. At the mouth of the canyon are smoke trees, ocotillo in scarlet bloom. We set up base camp here. White sand and golden boulders lead back into a tangle of thorny mesquite and catclaw bush. Wildflowers bloom here: satiny white desert blazing star, silken pink-white evening primrose, ultramarine blue Canterbury bells.

Farther in, wind- and water-etched walls of stone tower above the canyon floor. The cascading trill of a canyon wren's song from somewhere deep upcanyon tempts me to dare the boulders and the thorns. The harsh wheeze of a scolding gnatcatcher makes me think of a warning sentinel: intruder alert! You imitate the tiny bird's scold, tease it to flit to an open perch, tail flicking angrily, for an instant before it's gone again.

You grin, and I wonder if I was just imagining the other stuff, going too crazy myself with university politics and grant paperwork. "Let's get set up."

Sombra Canyon is open to the public, part of a national wildlife preserve, but you have to have a permit to bring a vehicle up the road, so only rare, brave birders or hikers come here. It isn't known for its rarities, except the hummingbird. And there's no guarantee of seeing those.

So we're alone, except for one guy camped a ways off on the plateau above the canyon. He has a scope, but it's pointed at the sky. There's something like a small satellite dish up there, hooked up to something else, probably a laptop, and a mini solar panel. Star-watcher, I guess.

I help you set up mist nets in the canyon mouth and near the ocotillo. There's a Black-Chinned Hummingbird working the red ocotillo blooms now; that's a good sign. The green-tails will come.

I hope.

We make love in the tent with the warm wind blowing in through the open flap, and you see me, you call my name and I bury my face in your shoulder, and everything is the way it should be, the way it used to be.

After dinner (you hate freeze-dried food; you make a concoction of lentils and curry spices while I dig a couple of slightly warm beers out of the cooler), we check the mist nets, and release about ten birds, none of them green-tails.

The wind sings deep in the canyon.

The moon rises, white over the hills.

In the morning, we head into the canyon. Our plateau-top neighbor spots us and waves. We wave back. Through binoculars, he's a middle-aged guy in a Griffith Observatory T-shirt.

There's no breath for talking, with scrambling over water-smoothed boulders, dodging catclaw, checking hand- and footholds for snakes. I think about the hummingbirds.

Of course, they weren't originally confined to one canyon. Up until twenty or so years ago, people observed them across the Southwest, never often, but regularly. It's thought they must be extremely sensitive to some aspect of human disturbance, even though hummingbirds are mostly tough little bastards, happy in human yards, swarming our feeders. Or maybe they feed only on some vanishing nectar, though no unique plant has ever been recorded here.

No one knows why they're here, and only here, and I know how desperately you want to find out. I can see it in the tension of your shoulders as we scramble farther in.

I want to know, too, but I don't think I want it the way you do.

We come to a seep where water trickles from broken rock and pools in the sand. There are birds here: White-Winged Dove, little black-faced Lawrence's Goldfinches, flitting warblers like tiny golden gems. There's desert bighorn poop and coyote scat. You sit silently with your back against a rock, watching, still as a wild animal yourself. I join you.

Above the seep there's a chuparosa bush with its little red trumpet-shaped flowers. Hummingbirds love those. We watch it.

The shadows cool and the rock turns a deeper gold.

After a long time you shift and sigh, and I reach for my pack, and then it's there, a male, his black feathers impossibly iridescent in the sun, his pale green tail plumes bobbing. His wings make a metallic whir. He feeds on the chuparosa for a while, and then he's gone, darting away, seeming to just vanish.

We jump up, high-five, hug each other. "You know what this means," you say at last.

It takes me a second to realize. "Shit."

It takes us all of the next day, a lot of cursing, and a lot of blood lost to catclaw bushes to move the mist nets and the banding gear up to the seep.

At least one of us has to be there all day, every day, after that. We can't let stressed birds struggle in the net. Two days later, we catch a green-tail. We weigh her and carefully place the tiny metal band around her fragile leg—she feels like electrified silk in my hands—and she darts away.

"She doesn't look stressed," you say, as the bird returns to feeding. "I don't know if human-intolerance is going to be our answer here."

We take down the nets for the night and head back to base camp for a celebration: the last beer.

We've just finished washing up, and I'm about to suggest you should leave your notes and come in the tent with me for more celebration, when someone says, "Hello, the camp!" just outside the circle of light cast by the lantern you've hung from the low branch of a smoke tree.

I feel more than hear you sigh, but you call out cheerfully, "Come on in!"

It's the star-watcher from the plateau, who, in close-up, is fiftyish, half-shaven, and carrying a bottle of Scotch. "Hey, guys," he says, "I'm Padgett. Been out here a couple of weeks. You two doing some kind of wildlife research?"

"Yeah," you say, and start explaining. I interrupt you to introduce us, then let you and Padgett talk while I sit back against a rock, sipping Scotch from a plastic cup. I'm tired. I feel like the project is finally getting somewhere. I let myself start thinking about a joint publication in Nature. Padgett's words and yours drift in and out of my attention.

"You're the first people I've told this to," Padgett says, "and you'll probably think I'm crazy, but I believe I have evidence of a discontinuity in the canyon. Some of the instruments show unprecedented readings. . . ."

You chuckle. "I'd have more of an opinion if you're crazy or not if I knew what a discontinuity was."

"A boundary, I guess you'd call it," Padgett says. "A place where a parallel universe touches our own."

I hear the sudden intentness in your voice when you say, "Explain." But I'm tired, happy, and drunk, so I just stumble over, kiss you goodnight, wave at Padgett, and crawl into the tent.

I can still hear bits of conversation. I've seen something like this on TV, this thing about the universe being just a bubble in an ocean of other bubbles, parallel universes, infinite. Maybe very different from our own, maybe almost the same. Padgett starts talking about the Big Bang and an eleventh dimension. "Everything in our universe," he says, "is a connected part of one membrane."

"I guess," you say, "but I've always thought there was no way to perceive these theoretical other universes? Much less to get there?"

"Ah," Padgett says, and I think sleepily, What a dork. "The eleventh dimension is turbulent. Universes can collide. Can touch. Come up some time and look at my instrumentation and you'll see . . ." Come up and see my etchings, I think. Whatever.

I fall asleep.

"It's killing them," you say. Bleakly, staring at the mist net, where a bird struggles: male, unbanded.

In four days, we haven't recaptured one of the seven banded birds, or even seen one a second time. Without that, we have no way of tracking their movements, learning their behavior, what territory they need, who's mating with whom.

I untangle the hummingbird, holding him gently. A tiny black eye blinks up at me. "Hon, that's crazy. There's no way banding them could kill them. Come on, help me with this guy."

Weighed, banded (black/red), the gorgeous bird whirs to scarlet flowers and starts to feed. "Look," I say. "He's feeding, he's not really even avoiding us. He's not stressed."

"No, but . . ." You run a dusty hand through your sweaty hair, leaving it standing on end. "Over time it's too heavy, it inhibits their flight. . . ."

"They're the same size band as for any other hummingbird."

"OK." You bend over for your pack, fumble with the zipper, get out a water bottle and drink. "So why, in four days, haven't we seen even one of the banded birds again?"

I hold out my hand for the water. The sun glares off the canyon walls, the sand. "They're super-intelligent? They don't let themselves get caught twice? God, Dave, how the hell should I know?"

"Not just not caught twice." The little male zooms over our heads in a flash of green plumes. "Never seen twice." He's gone upcanyon. Vanished. Like the rest.

"A migration no one's recorded. A seasonal movement . . ."

You look up at the cascade of a canyon wren's song, and I stupidly hope you'll smile and imitate it, but you just say, "If we don't solve this right away, the project is a failure."

"We need to capture one," I say. "I know you don't like it. But we have the permit, we have the gear . . . we need to see what's happening."

"If we have to," you say, but your boot-toe scuffs in the sand.

That evening, after dinner, you head up the steep trail to Padgett's plateau, and you don't come back till deep in the night.

The next day, dusk. The edge of the sky is turning amethyst. When I hear your footsteps, I spin around. "Are you OK? What took so long?"

You're scratched bloody, bruised, your shirt dark with sweat. You just shake your head. "I went all the way up. The canyon ends in a talus slope. I scrambled up to the plateau. Nothing."

"You didn't see any of them?" I wave an early nighthawk away from the nets; it tilts higher into the evening sky.

"I didn't see a single green-tail. Or . . . anything else of interest." You sit—no, collapse—on a rock. "I thought . . . you know, Padgett gave me this dumb idea the canyon was magic or something. If I could just get through to the end, I'd see them. I'd know.

"It's just a pretty canyon. There's another seep. Couple of bighorns up on the rim. It's not . . ." Your voice catches. I've never heard that before. "I don't understand how they're doing this."

"I caged a female today. She's back at camp," I say, and sit down on your rock, against your sunburned, thorn-slashed shoulder. We wait there together for night to fall.

But when we get back to camp, stumbling in the moonlit half-dark, you don't say anything to me. The captured female sits quiet in her cage with its blooming sprigs of chuparosa and ocotillo. At first, she whirred frantically up and down, but she never smashed into the wire. Now, after nightfall, she's asleep, but you stare into the cage for a long time, and then you start up the trail to Padgett's camp. I watch you go. Something moves coldly inside me.

"No green-tail for three days," you say grimly, turning on the camp stove for coffee. Trying to. It sputters, rocks, suddenly roars to life cranked way too high. You snatch back your hand. The stove is a ball of flame. I grab for the wash water. Before I can get there, you kick the stove into the sand at the base of a rock. Its burning bits scatter and go out.

"Dave? What the fuck?" I should say, Are you OK? But all I can think is, You just kicked a flaming ball of gas across a bone-dry desert wash when all you had to do was turn a knob.

"I . . ." Dark circles undercut your eyes. "I forgot which way to turn the gas spigot."

The captured female says chip! from her cage, but when I look over, she's feeding, showing zero concern for our human incompetence.

"OK," I say, "OK, we didn't really need a stove, because we are out of here. Better than us have failed with the Green-Tailed Hummingbird. It's not the end of the world. I'll go pack up the nets."

"No."

"We're getting nowhere, and you're—"

"Heather." You walk over, take my hands. A blister is rising on one of your fingers. Blue eyes. You seem to see me. "What if they're going somewhere we don't know how to follow? What if it's not just these, but Ivory-Billed Woodpeckers, Eskimo Curlews . . . What if we've made the world into somewhere they don't want to be, and they're going somewhere else?"

"This is Padgett's parallel universes crap. You need to quit this project, Dave."

You start toward the cage. "I'm letting her go."

"No!" Even though I desperately want this project to end. I take a breath. "There's more to learn still. Look at her, she's fine, she's eating."

You turn on your heel and start upcanyon.

"Hon?"

You don't turn around.

I hike up the trail to the plateau. I want to scream at Padgett for giving you this idea. This obsession. But his camp is gone.

I descend in the heat. Summer is coming. We've been here too long. A rattlesnake warns me from a brush pile behind camp.

The hummingbird perches in her cage. There's nothing obviously wrong with her. An animal with a brain the size of a pencil eraser, no matter how brilliantly adapted, can't feel sorrow.

But she seems to grieve.

I wait for you. Hours, and for a long, chilly while I think you may not come back. But you appear out of the rocks at last. "Dave," I say.

You half-wave and drop your pack by the tent. I see the straps are twisted, like you didn't buckle them right. "Dave."

"Yeah. . . ." Still not looking.

When I say your name the third time you look at me: blue eyes. I open the cage and the hummingbird flashes free, turns a loop I can't call anything but joyous, and is gone.

Late that night you turn over and reach for me. Our skins are fever on fever in the June night. Your grasp leaves bruises. I call your name.

"It's not you," you say, half-choked. "I love you. You're not the reason."

There is a moment—maybe—when I could reach out and grab your hand as you pull on your torn, dusty shorts. When I could call your name.

I let you go.

Footfalls on sand. Then gone.

It's May, and in Sombra Canyon the songbirds flutter around the clear water seep.

No one has seen a Green-Tailed Hummingbird in almost exactly a year.

I sit on a water-smoothed rock near where we set up the mist nets last year. Warm wind touches my face.

The canyon wren sings. For an instant, I think I hear your whistle echoing its call.

Somehow, there was a door in the cage of the world.

I get to my feet and walk back down alone.

Anza-Borrego and Death Valley

2009