For nine months of migration each year the curlews were the pawns on a great two-continent chessboard and the players that decided the moves were the cosmic forces of nature and geography—the winds, tides and weather. Winds determined the direction the birds would fly. Tides and rainfall, by controlling the availability of food, determined each flight′s goal. Now another player, an ocean current, entered the game.
The Pacific′s massive Humboldt current, which sweeps northward from the antarctic along South America′s western coast, carries chilled water almost to the equator. The onshore breezes that each afternoon blow in to the Peruvian coast are dry winds for there is little evaporation of moisture from the cold Humboldt water. So the narrow coastal strip between the Andes and the sea is a parched region of sandy desert plateaus where rain rarely falls. Few rivers tumble down the Andes′ western slopes into the Pacific to create the estuary mudflats on which the tides can scatter the foodstuffs of the sea for the shorebird flocks. So here the shorebirds eat sparsely. They are tired and thin after the high Andes crossing but the coastal deserts conceived in the Humboldt′s antarctic water drive them on without rest.
The curlews followed the narrow Peruvian beaches northward, flying hard each night until the dawn, using every hour of daylight in the wearying search for food. They were always tired and never fully fed. There was neither time nor energy now for the courtship displays, little time even for rest.
In less than a week they covered two thousand miles and reached the sandy flatlands of Punta Parinas near the equator, where the South American coast turns back northeastward toward its juncture a thousand miles away with the Isthmus of Panama.
March was almost here. Far to the north, spring would be moving up the Mississippi Valley, greening the cottonwoods and prairie grasses. The curlews were still south of the equator, the tundra was still six thousand miles away. Now the arctic beckoned with a fever and fierceness that their aching and wasted breast muscles couldn′t still.
Here the coast swung in a great twenty-five-hundred-mile crescent east, north and west to the rich highlands of Guatemala, but straight north, across the bight of the Pacific enclosed by this crescent, Guatemala was only twelve hundred miles away. The male curlew was still hungry, his crop half-filled, when night began cooling the hot sands of the Parinas desert. He climbed into the tropic twilight and the female followed close behind. And he turned north, away from the low coastland, out into the Pacific where the landfall of Central America lay twenty-four hours of flying away.
They flew silently, wasting neither breath nor energy with calling to each other. It would be an ocean crossing only half as long as the exhausting autumn flight down the Atlantic from Labrador to South America, but the crowberries of Labrador always assured that the autumn flight could begin with bodies fat and fully nourished. Now they were wasted and thin. In two hours their stomachs were gaunt and empty again.
The southeast trades were left behind after four hours of flying and they entered the calm, windless area of the doldrums at the equator. But the sea below was far from calm. It danced wildly in small, steep waves with foamy, hissing crests—a battleground of waters where the cold Humboldt current met the warm flow of the equatorial current and battled for possession of the sea. Then, even by moonlight, they could see the ocean′s color suddenly change as they left the cool green Humboldt waters behind and flew on over the deep blue of the equatorial sea. The air became warmer abruptly.
The moon set and the dawn came. Shortly after dawn they reached the region of northeast trades, a crosswind that made flight easier. But the day rapidly turned hot and the stifling, humid air soon canceled the benefit of the wind.
They flew hour upon hour, the speed of their wingbeats never varying for a moment from the monotonous, grueling three or four beats a second. The glaring sparkle of the sun on the water diminished and finally disappeared as the sun approached its zenith. The sea turned a deeper blue. Then the sun dropped toward the west, the sparkle returned to the wave crests half a mile below and the air grew hotter still. Since the South American coast had disappeared in the darkness of the night before, there had been nothing to break the flat emptiness of sea except an occasional albatross gliding on gigantic, unmoving wings. But the curlews flew northward unerringly, never deviating, their brains tuned more keenly to the earth′s direction-giving forces than any compass could be.
The male, partially breaking air for the female, was suffering greater fatigue. The sharp, periodic pains of his breast muscles had changed to a dull, pressing, unabating ache in which he could feel his heart thumping strenuously. He could have obtained some rest by moving back and letting the female lead, but the realization that she was close behind, drawing on the energy of the air that his strength produced, her flight a dependent part of his own, was a warm and exhilarating thrill that made him cling staunchly to the lead position.
The sun went low in the west and his strength dropped to the point where no amount of stubborn mental drive could keep his wings working at the old harrowing pace. But still he clung to the lead. His wingbeat slackened and the flight speed dropped. As soon as she noticed it, the female, who had been silent for almost twenty-four hours, began a low, throaty, courtship quirking, and it gave him strength as no food or rest could do. She repeated it at frequent intervals, and the sun dropped close to the horizon sparkling the sea with a million golden jewels of light, and their wings drove them endlessly on.
The sun was setting when the hard blue of the sea at the horizon ahead of them became edged with a narrow, hazy strip of grey-blue. For several minutes it looked like a cloud, then its texture hardened, and behind it higher in the sky emerged the serrated line of the Guatemalan and Honduran mountain ranges. The outline of the distant volcanic peaks sharpened. The lowland close to the sea changed from blue to green, and a white strip of foaming surf took form at its lower edge. There was still a half hour of daylight when the curlews reached the palm-fringed beach. They commenced eating immediately. When darkness came the pain of hunger and fatigue was already diminishing.
They fed busily all next morning, but the feeding was not good for the beaches were scattered and narrow, and swept clean by the Pacific′s surf. By noon the day was very hot, but the curlews flew again. They flew inland now, for this was the Central American summer and the grassy highlands of the interior would have a rich crop of grasshoppers. They flew across the coastal plain, which rose gently into the mountains behind. The black fertile volcanic soil was thickly covered with breadfruit trees, coconut palm and plantations of banana trees and sugar cane. In an hour they had climbed a mile above sea level, moving suddenly from tropics to a temperate zone where the air was dry and cool. They climbed higher into mountainous country, then entered a narrow valley that led them through to the rolling tablelands beyond.
They flew four hours and finally landed on a hilly plateau two hundred miles inland from the Pacific. Here, for the first time, the curlews joined the hosts of migrants that were flowing northward to overtake the North American spring. In the forested valleys were swarms of tanagers, thrushes and warblers, all feeding busily to store energy for the long night flights. On the grassy uplands were flocks of other shorebirds and bobolinks. But there was no bird song, for song was the proclamation of the breeding territory, and the breeding territory for most was still two thousand miles away.
On the sloping hills grasshoppers swarmed everywhere. The grass was trampled and cropped close by great herds of sheep, so the insects were easy to find. The curlews fed until their crops and stomachs were gorged. With nightfall thousands of other migrants began passing overhead. Except for the occasional one that passed in silhouette across the face of the moon, they were hidden in the dark, but their lisping chorus of flight notes was an uninterrupted signal of their passage. But the curlews waited, for winter still gripped their arctic nesting grounds and here they could fatten for the final dash north.
They waited a week, feeding well, straggling slowly northward each day. Their bodies grew firm and plump again and with the return of strength the mating urge burned like a fever within them. By the end of the week they had moved out on the Yucatan peninsula to its tip. Five hundred miles northward across the Gulf of Mexico were the swampy shores of Louisiana and Texas, with nothing beyond but the flat unobstructed prairies reaching almost to the arctic.