High-Tide Rush Hour
By Awake! correspondent in Britain
ABOUT ten million birds winter in northwestern Europe each year. They come not only from arctic breeding grounds but also from as far afield as Canada and central Siberia. On their way to Africa, many others converge on the East Atlantic Flyway, a migration route across the British Isles.
Food and havens for rest are provided by a chain of over 30 large estuaries in British waters. Each of these caters to more than 20,000 birds, but the most important is The Wash, on England’s east coast, which hosts more than a quarter of a million birds—including curlews, dunlins, godwits, knots, oystercatchers, plovers, redshanks, and turnstones. What kind of food do estuaries provide, and why are they so important?
The Importance of Estuaries
Estuaries are semienclosed coastal areas where sea water mixes with fresh water. Here the warm waters, rich in both mineral and organic nutrients, sustain one half of the living matter of the world’s oceans. Shrimps, sand hoppers, and other forms of life are found in sand, but estuarine mud sustains an even greater abundance of life.
Mud varies according to the size of sediment from which it is made. Each mud type contains its own special marine animals, the food consumed by waders.a For example, in a square yard of one kind of mud, there can be millions of tiny snails that are less than an eighth of an inch [3 mm] long! In addition, mud nurtures mollusks, lugworms, and rag worms, along with other invertebrates.
Spring Tides
Even though there can be many thousands of waders on an estuary, it may be hard to locate them because they are usually scattered over vast areas. When the spring tides arrive, however, the situation changes dramatically. The surge of high water causes the sand and mud flats to become inundated, forcing the waders onto the saltingsb and other higher locations. It is then much easier to observe them as they mass together, roosting in huge mixed flocks.
Today, a bright, sunny April morning, a spring tide is due. A cold northeasterly wind is blowing as we drive to a small, picturesque estuary where the river Alde meanders through the English county of Suffolk into the North Sea. Here, the winter wader population peaks at just over 11,000, and we will find it much easier to observe their activity, as the estuary is only half a mile wide.
A complex of broken sea defenses follows the course of the river. Some banks are covered with reeds, others with marram grass. The remainder consist of bare black timberworks and stone. Just upstream, amid a remarkable collection of Victorian buildings, is the Snape Maltings Concert Hall, home of the Aldeburgh music festival. But we must walk downriver and head for a sheltered spot. The wind is now strong and biting, and our eyes are soon smarting.
As soon as we arrive at the river’s edge (see picture, point A), we are greeted by the clear, liquid call of a pair of avocets. They are no more than 40 yards away, on our side of the estuary, presently engaged in mutual pair-bond preening. Each bird nibbles the sides of its upper breast with the tip of its slender, upturned bill. This is delightful to watch, but we must press on, as there is much more to see.
The Rising Tide
The tide is rising quickly now, so we move smartly to our chosen observation point. (See picture, point B.) On the way a redshank—living up to its reputation as sentinel of the estuary—lifts off from the saltings, with its shrieking call of alarm, “tuhuhu-tuhuhu!” Its red legs contrast with the brilliant white trailing edge of its wings glinting in the sunlight. Upon arriving at our destination, we quickly scan the rapidly shrinking sand and mud flats.
In the distance a score of redshanks feed steadily, lightly probing the mud, while others search for food in more sheltered inlets. Dunlins, with their characteristically downturned beaks, stick closer together, in small groups. In straggling line they hurriedly pick their way along the mud, tending to keep close to the water’s edge. Scattered curlews amble by, carefully probing the soft, slimy ooze. Farther upstream, a couple of turnstones are locating food by using their short, slightly upturned beaks to roll back tide-line debris on the old sea bank.
Suddenly, the wild, wistful, trisyllabic whistle “tlee-oo-ee” of the gray plover fills the air. As it flies overhead, the bird’s black axillaries show up clearly against the rest of its pale underside. Four hundred golden plovers, closely packed in an oval formation, rest with head under wing, all facing the wind. There is the occasional dispute between a couple as pecking order is reaffirmed. The majority are still in their speckled winter plumage—with gold and dark upper parts; pale coloring around the eyes, face, and underside; and a black beak. As we pan our telescope, we spot ringed plovers too.
A widespread flock of some 1,000 lapwings suddenly arrives. The birds approach in gay abandon, flopping through the sky in their inimitable way. The lapwings and the golden plovers have been on the arable land to the west, their more favored feeding zone. They come to the estuary not just to feed but to bathe and preen their feathers.
The main background noise is from the bubbling call of curlews, the more contented musical whistle of redshanks, and the squawking of black-headed gulls. Two bar-tailed godwits probe deep into the mud. With their thick orange-red bills, a few oystercatchers are pulling out lugworms. A lone gray plover takes a few stately steps, stops, shakes the right foot, then goes after its prey, and swallows. But the incoming tide is quickly overtaking all of them!
The Rush Begins
Suddenly, the birds lift off to form flocks, mainly of their own species. It is an impressive sight, for the waders fly in tightly packed formation. Banking from side to side, flocks change color as the sun’s rays catch them—from dark brown to shining silver-white—one moment clearly visible and the next, almost merging with the background of the muddy incoming tide. Dark to silver, silver to dark, in perfect rhythm and, at the same time, constantly changing shape—from roughly oval to round, then to a spiral, and finally to a vertical line. Most drop back to mud flats yet to be covered by the tide.
Soon, the mud and sand flats around us will be inundated, so we hurry upriver, accompanied by a steady stream of waders. First to overtake us are small flocks of tiny dunlins, with their fast wingbeats, maintaining contact by occasionally uttering their short, shrill whistle. Then, the bigger redshanks pass, their flock more spread out and stately. Curlews the size of large gulls fly by, fluting their lovely, bubbling mellow trill as they go. Avocets follow in one big flock, contrasting black and white against the blue sky. They settle at the top of the estuary, their long, slate-blue legs just visible above the water.
The Roost
We quicken our pace to reach an elevated point where the estuary narrows. (See picture, point C.) Species tend to flock together, though this is clearly not a rule. As the tide continues to rise swiftly, more birds join the throng. This causes constant reshuffling as standing room becomes more difficult to find on the banks, with an ever-increasing demand for space from those arriving late.
The high tide is upon us now. The lapwings and golden plovers have flown back to the arable land. All the remaining birds have been forced off the mud to roost on the old riverbanks. The constant piping of the oystercatchers is all out of proportion to their numbers. The redshanks and curlews add to the background clamor, which is now transcended by a skylark singing overhead—a truly wonderful atmosphere.
We take our leave as the waders enjoy their well-deserved afternoon roost, sitting out the high spring tide. In spite of the fact that some are behind the seawall and cannot see the water, the birds will know when to return to their mud flats or sandy shores. Perfect timekeepers, instinctively wise, they know how the tides work.
Yes, a high-tide rush hour is thrilling to watch, especially for the first time!
[Footnotes]
a In the United States and Canada, waders (order Charadriiformes) are better known as shorebirds.
b Land flooded regularly by tides.
[Box/Picture on page 26]
Enjoy the Rush Hour
To enjoy a high-tide rush hour, first of all locate a convenient estuary. You will then need some information about the area, such as where the waders go and where to view them. Consult tide tables for a high spring tide that occurs just after a full or new moon. In addition to travel time, allow three hours for a good look at the birds, and arrive at least two hours before high tide.
What equipment will you need? If you are unfamiliar with waders, take a book to help with identification. A pair of binoculars can be very useful too. You will soon learn that each species of wader has its own characteristics and collects food in the manner for which its beak was created. A telescope is not essential—but warm, waterproof clothing is! Be alert to dangers. Do not venture out on mud flats unless you know them well. It is easy to get trapped by a rapidly rising tide. Furthermore, it takes only a sea mist to drop for you to get lost. Consider the wind, as well. Gales can cause surge tides, which can be very hazardous on any estuary.
[Box/Picture on page 27]
Main Global Estuaries
The Wadden Zee, in the Netherlands, is the most important intertidal area in Europe and at times holds probably over four million waders. It stretches northward to southwest Jutland. Three good locations to visit in this vast area are the causeway to Rømø, in Denmark; The Weser River estuary, a major high-tide roost, in Germany; and Lauwers Zee near Groningen, in the Netherlands. On the Iberian Peninsula, the most significant estuary is that of Portugal’s Tagus River.
Estuaries along the Pacific seaboard of both North and South America provide food for some six to eight million wader migrants. Among the main locations are San Francisco and Humboldt bays, in California; the 80 square miles [200 sq km] in Canada from Vancouver’s Boundary Bay around to Iona Island, British Columbia; and Alaska’s Stikine estuary and Copper River delta.
Excellent sites for waders are also to be found at Bolivar Flat and Galveston, in Texas, U.S.A.; at Tai-Po, in Hong Kong; at Cairns, in northeastern Australia; and near Mombasa, Kenya.
[Picture on page 24]
Five oystercatchers
[Picture on page 25]
Knots rushing from their roost
[Picture on page 25]
ALDE ESTUARY, SUFFOLK
Snape Maltings Concert Hall
Observation point B
Viewpoint C
Initial sighting A
[Credit Line]
Snape Maltings Riverside Centre
[Picture on page 26]
Knot
[Picture on page 26]
Redshank
Curlew
[Picture on page 27]
Above: Curlews