The dirt road leads west from Nineteenth Street in Imperial Beach, about a quarter mile of rough, bumpy driving that ends at a small turn-around. Ahead lies a cultivated field, but a path curves around the side of it toward sandy hillocks and the beach. This is the Tia Juana River mouth, the southernmost part of the California coast.
To the north are the yellow hangars of a Navy auxiliary landing field; three miles south stands the wire fence that marks the border with Mexico. For the last few years this place has been the site of a dispute between environmentalists and developers, the latter of whom proposed to build housing and a marina in the lowlands near the river. Now it appears that the federal government will purchase the land for use as a wildlife refuge, thereby preserving what is one of San Diego County’s prime bird habitats.
A pale sun rises over the freeway to the east as I take the path alongside the field. Tractors sit unused this Sunday morning; a meadowlark breaks into clear, cheerful song from atop an empty oil drum. After a few hundred yards the path begins to parallel a narrow water channel that now, at low tide, is filled with birds probing the mud for snails and crabs.
Their drab and somewhat similar plumage belies the many different species: willets, dunlins, sandpipers, marbled godwits, and long-billed curlews, among others. It is only September, but many of these birds have arrived for the winter and would not have been found here a few months or even a few weeks ago.
The path leads onward toward the beach. Imprinted in the damp ground at my feet are thousands of different bird tracks, some curving away, others looping back, intersecting in a maze of patterns. They grow particularly thick at the edge of a small inlet, where the path abruptly ends. At my approach two godwits feeding nearby take to the air with a sharp cry of alarm; their pale brown wings look rose-colored in the morning light as they disappear southward.
Suddenly there is a great commotion from the other side of the inlet, and a huge flock of elegant terns – a gray and white bird with a black cap and orange bill — starts up. In a few moments there are roughly a thousand of them flapping and calling overhead, making a tremendous din. Far away across the sand another flock takes wing; they turn this way and that in the distance, presenting a glittering effect of white, then gray, then white.
Only in the fall are elegant terns found along San Diego’s coast in such great numbers. Although a few pairs nest locally, the vast majority breed to the south in the Gulf of California. But in a phenomenon characteristic of many species, they fly north to gather in flocks before making the long journey south to the coasts of Central and South America for the winter. No one knows exactly why elegant terns gather in San Diego, or how they know where to stop, or what prompts them to leave their breeding grounds in the first place. It is all part of the complex and massive shifting of bird populations known as the fall migration.
Dr. Amadeo Rea, curator of birds and mammals at the Museum of Natural History in Balboa Park, is a wiry man with a full black beard and pale blue eyes. In the last twenty years he has collected and analyzed bird specimens from all over the western hemisphere. The shelves in his office at the museum are littered with feathers and bird bones with tags attached to them, and there are likely to be several long-dead specimens arranged rather forlornly on top of the papers which cover his desk. Rea is a taxonomist who captures and studies different bird species and subspecies as a means of learning about their migratory habits. He often finds it necessary to shoot these specimens in the field in order to be able to study them properly, but he chooses his subjects carefully and avoids the destruction of birds he does not need. Rea is only thirty-nine years old, but his hair is flecked with gray.
“The migratory habits of birds today probably developed about three million years ago,” he told me in his office on a sweltering afternoon recently. "The movement of glaciers at that time changed the availability of food, and migration likely began as a means of discovering new food sources.”
How birds know when to migrate isn’t precisely known, but they appear to react to the length of the daylight period as well as physiological changes in their own bodies. For many birds the latter means a change of plumage. “Here, I’ll show you,” Rea said, sliding open a cabinet drawer to reveal neat rows of dead specimens. “This is a scarlet tanager I took on the Amazon a few years ago.” He handed me the soft, nearly weightless body. “You can see that the olive green color of this bird is a far cry from its scarlet breeding plumage. It hardly looks like what we know as a scarlet tanager at all. But bright plumages are primarily a means for birds to identify their own species during breeding season. In the winter there’s no need for that, so some birds molt to a less colorful plumage which, incidentally, serves to make them less visible to predators.”
Rea had warmed to his subject and began gesturing with his long, bony hands as he talked. An Indian bracelet of silver and turquoise gleamed on his wrist. “Species like the elegant tern that fly north in the fall probably do so to take advantage of food sources available in certain areas. How they discovered there was food to be had in those areas, though, isn’t known. They simply evolved traditional migratory routes that now appear to be passed on from generation to generation genetically. And these routes vary widely in length and direction. Keep in mind, there are different types of migration.”
Some birds, he noted, breed in the north and migrate in the fall to the tropics. Other birds breed in the north and migrate only to temperate areas like San Diego. Still others – locally, the robin would be an example — simply move down from higher altitudes to lower ones for the winter. And some birds, of course, don’t migrate at all.
As might be deduced from this, all birds do not migrate at the same time. At almost every time of the year there is some species migrating somewhere. Wilson’s phalaropes fly south as early as mid-June, a time when willow flycatchers are just arriving in San Diego for the summer. Among the later fall migrants are some species of hawks, who don’t begin their migration until October. At that time they can be seen gliding high above coastal ridges like the one along Point Loma, taking advantage of wind updrafts as they move southward along the coast.
How do birds know where to go? Rea’s eyes shone as he pondered the question. “Well, we don’t really know all the answers to that. Some birds migrate at night, and it’s been proven that these species navigate by the stars. Exactly how they do it, though, no one is completely certain. In the Americas we’re really just scratching the surface on a lot of these questions.”
Occasionally a bird will fly hundreds, perhaps thousands of miles off its usual migratory path. If someone spots it while it is in this unfamiliar habitat, it becomes known as a “rare bird,” and there are birders (as bird watchers sometimes refer to themselves) who will travel hundreds of miles just to catch a glimpse of it. According to Rea, a bird that has flown off of its migratory path is possibly lost. “Migrating birds can get lost during prolonged overcast periods when they can’t orient themselves to the stars or the sun. But it seems to me more likely that many of these birds have imperfectly received the genetic code for their migratory route. A code like this is an enormously complex thing, and one small variation would mean that the bird might end up on the West Coast instead of in South America.
“Migration is a real hazard for birds,” he continued. “Some of the biggest killers are lighthouses, TV and radio towers, tall buildings. There are buildings in the eastern United States that kill hundreds of migrating birds each night. It’s a matter of sweeping them up in piles in the morning.
“A migrating bird also takes its chances in finding suitable places to rest along the way. Man is drying out and ruining many habitats, and this seems to me to be one of the long-range problems in bird migration. Where will they stop? What will they do if all the right habitats are gone — just keep on going? Birds have to refuel, but what happens if there are no more gas stations for a hell of a long way?”
Early morning clouds hang over Agua Hedionda Lagoon in Carlsbad. About forty people armed with binoculars and telescopes stand near the water’s edge; most are Audubon Society members who have gathered here for a bird-watching tour of San Diego’s North County lagoons. These lagoons – Agua Hedionda and Bataquitos in Carlsbad, and San Elijo in Cardiff — are a good place to see migrating shore and marsh birds, mainly because they are some of the last undisturbed habitats left on the Southern California coast.
The leader of the tour is Phil Unitt, a husky young man with blond hair, a ruddy complexion, and gold-rimmed glasses. Earlier this year Unitt began a study of the least tern, an eight-inch long, gray and white bird that has been on the federal endangered species list since 1970. The study, sponsored by the California Department of Fish and Game, required someone with an extensive knowledge of birds to count the least terns nesting locally and record their breeding success. Unitt is only twenty-two and just received his bachelor’s degree in zoology from San Diego State University this spring, but he has been interested in birds since he was fourteen and is one of the top two or three birders in San Diego.
Right now he is standing with binoculars to his eyes, staring out across Agua Hedionda Lagoon. Most of those present, who range in age from fourteen to about sixty, are wearing hiking boots and jackets as protection against the morning cold; Unitt has on only jeans, a blue flannel shirt, and a pair of mud-caked green and white tennis shoes. As he scans the lagoon he identifies birds which to the less experienced observer, even through binoculars, appear only as a vague dot in the distance. The rest of the group surveys the water with varying degrees of success.
“What was that cinnamon-colored bird that just flew over?” asks one young man.
“That was a marbled godwit,” says Unitt.
“I think my husband just saw a doughnut and coffee fly over,” whispers one middle-aged woman to another.
Two boys spot a great blue heron standing on a point of land not far away. “We’re in the ninth grade,” I overhear one of them tell an elderly lady a few minutes later. “Our teacher is on the tour, too. He announced in class that any students who were interested could come along. We’re the only two who showed up.”
Unitt shows the heron to those who have not seen it, and points out nearby willets, sandpipers, and a small flock of black-bellied plovers resting on a sandbar. All of these birds, he explains, are winter visitors to this area. Then, in response to a question, he begins to talk about his work with the least tern.
“At this lagoon we found ten pairs nesting on the mud flats a little bit east of here. These twenty birds managed to fledge only four young; you couldn’t call that great success. One of the problems was that the nesting area was accessible to motorcycle riders who came through a gap in the fence nearby. I don’t think the motorcycles actually ran over any eggs, but the noise alone would be enough to scare the birds off.” Someone ventures the argument that least terns are disappearing simply as a result of natural selection, and Unitt smiles ironically. “The least tern wouldn’t be endangered if it weren’t for the fact that human use has taken over many of their nesting sites,” he says. “They come back one spring and find that their nesting area has either disappeared or is now suddenly accessible to a large number of people. The birds move on to someplace else or fail to nest that year. Obviously, if they fail to nest for a number of years, the population is going to be reduced.”
He goes on to say that least terns have now left San Diego for the winter. No one knows where they go. It’s thought that they winter on islands off the coast of Central or South America, but with the estimated total population only 775 pairs, no one has been able to trace them.
While Unitt has been talking, the great blue heron has stood unmoving on the point, seemingly impervious to both the group of people watching it and a highspeed boat towing a water skier that periodically passes close by. Now, as we head back to our cars, the boat swings close one more time and the heron starts up and flies eastward with slow, sweeping strokes of its broad wings.
At Bataquitos Lagoon a few miles south, Unitt points out some of the more unusual winter visitors to San Diego County, including a white-faced ibis, a large brown bird with an extraordinarily long, downward-curving bill. It is feeding near reeds on the far side of a narrow inlet, eyeing us warily; finally it tires of being on display for such a large group of intruders and takes off for a more secluded spot. “There are six ibis that stayed all summer in this lagoon,” Unitt explains for the group’s benefit. “Normally they leave here around April and don’t come back until fall. Why did these six stay? It’s hard to say; it’s pretty unusual. I guess they just liked it here.”
It is nearly noon, and Unitt decides to bring the tour to an end before driving south to San Elijo Lagoon. Since most birds become less active during the middle of the day (in order to avoid the heat), today’s prime bird-watching hours are over anyway. But just before we head back to our cars, Unitt sights a small group of pectoral sandpipers, a medium-sized bird with rich brown markings on its back. “Certainly the most interesting species we’ve seen today,” he remarks, and by interesting he means rare. To the novice, even some of the common species may appear striking, but expert birders tend to frequent only those places where rare visitors are likely to stop. The North County lagoons are such areas, but they provide shelter only for birds that prefer marshy habitats. One must look elsewhere to find migrating land birds.
On another occasion I accompanied Unitt to a small canyon near the Tia Juana River known as Smuggler’s Gulch. The canyon runs east-west across the international border, and the ground was covered with bits of Spanish-language comic books and other signs of Mexican workers who may have stayed in the canyon before seeking employment, legal or otherwise, in the United States. The place wasn’t exactly a garden spot, but the willow and tamarisk trees there attract migrating warblers, and migrating warblers attract Phil Unitt. We crashed around under the trees awhile; they were so dense that the few birds that could be heard singing nearby couldn’t be seen at all. ‘To identify birds in a place like this you have to rely a lot on their songs,” Unitt finally admitted, then identified an unseen California thrasher and an ash-throated flycatcher. Unitt owns the three-record companion to A Field Guide to Western Birds, and he has spent many hours listening to it in order to learn to identify birds by their calls alone. Still, Unitt said the records aren’t nearly as valuable as going out and listening to birds in the field. “The problem with the records is that they often have songs that you’d hear only at a bird’s breeding grounds, which might be in the arctic.”
As we made our way back through the willows, a small greenish bird flashed by with a buzz. “Black-chinned hummingbird,” Unitt grinned. When we came out into the open at last, he turned back to look at the stand of trees and sighed. “You see a lot of good birds at this place,” he said, “but there are a lot of birds you don’t see."
We drove north along the Silver Strand "toward Coronado, and Unitt talked more about his study of the least tern. Forty-three pairs were found nesting at Lindbergh Field, on the sandy ground between the runways. The location matches their natural breeding grounds in every way except for the roar of jets going by. So far the birds have shown no ill effects from a decibel level that would damage human ears.
“There’s another nesting spot just a little north of here,” he went on, pointing to a stretch of beach just south of Coronado. “There might be a few late breeders left.” But when the place came into full view, Unitt groaned. The parking lot near the spot he had planned to visit was covered with the campers and motor homes of people spending a day at the beach.
“Do you want to stop?” I asked.
“I don’t see any reason to,” he replied, and his disgust was plain. “The least terns would be gone. There are people there.”
In late September I returned to the beach near the Tia Juana River mouth. A light autumn haze hung in the air above the sand and water. Beyond the breakers a school of black porpoises could be seen; only their dark backs and fins were visible as they swam slowly southward.
In the distance a flock of brown pelicans circled above the water a few hundred yards out from shore. Occasionally one would fold its wings and dive swiftly into the water in pursuit of a fish. The brown pelican is a big, stately looking bird, and the diving ones made a sizeable splash as they knifed into the water bill-first. It looked as if they were catching a lot of fish. Brown pelicans are an endangered species; when DDT was used extensively a few years ago, it eventually found its way into the birds’ bodies and weakened the shells of their eggs when they were laid. The birds crushed their own eggs as they tried to incubate them. With the banning of DDT, however, brown pelicans seem to be making a comeback. The nearest breeding colony to San Diego is on the Coronado Islands, but studies have shown that most of the birds here at this time of year come from the Gulf of California.
I walked north along the beach, expecting every couple of hundred yards to surprise at least one huge flock of elegant terns. I had been here not three weeks earlier and seen thousands of them. They’re a beautiful sight in their huge flocks, all flying and calling at once. It’s a wonder they don’t bump into each other, or maybe they do and just don’t mind. But on this day there was no sign of them on the sand and none were visible in the sky. A lone heerman’s gull, fighting the wind, glanced down at me impassively as it flew past. After two miles or so I finally stopped and admitted to myself the obvious: the elegant terns were gone. They had flown south for the winter, to gather in huge groups of 10,000 and more on the coastal islands of South and Central America.
I turned and walked back, tramping slowly through the dry sand away from shore. Through binoculars, a whitish area could be seen on the beach far away; perhaps a flock of terns, or perhaps – at this distance – even a wide patch of trash. But when I was nearer I could see it was a group of snowy plovers. They, too, gather here in flocks before flying south for the winter; and now, in late morning, they seemed to be patiently waiting for something. I smiled, because the snowy plover is a plump white bird with dark eyes and a short black bill, and waiting patiently looks like one of the things it probably does best. I drew a little closer, but the group seemed to notice me all at once and hurriedly ran off across the sand. Now and then they stopped to look back at me before moving off again, and the look in their eyes was one of terror.