Every year, thousands of small wading birds put on a spectacular show over the Norfolk shores.
The sun rises on a freezing cold morning at Snettisham on Norfolk’s west coast, as the tide comes in over the mudflats. Suddenly, tens of thousands of birds lift off as if as one, forming an undulating mass of wings and feathers passing directly overhead. Coming in to land at their roost site further up the shore, they huddle so closely together they could be mistaken for a bank of large, grey pebbles. Both in flight and on the ground, no daylight can be seen between them.This extraordinary behaviour is one of Britain’s greatest wildlife spectacles, performed by stocky waders called knots. Grey above and white below, they are generally inconspicuous birds, 10in (25cm) long with a 20in (50cm) wingspan. However, they make up for this nondescript appearance by their sheer numbers and their behaviour as the winter tides flow in and out.
The knots come in their thousands to winter on the Wash from breeding grounds in Greenland and Canada. The first few arrive during July, joined by many more through late summer and autumn. Numbers at Snettisham typically range from 40,000 to 80,000 during the coldest months. A record 120,000 came in the winter of 1990/91, an exceptional year. They leave again from March onwards, and by June they are all gone.
The Wash’s attraction is the huge expanse of mudflat exposed at low tide, seaward of Snettisham beach. Vast numbers of shellfish, small crabs and worms lie just below the mud’s surface. Most favoured item for the knot is a small clam, the Baltic tellin. The birds find them by probing intertidal mud with their beaks while pacing along the shore. In soft sediments, they may plough forward with the bill inserted into the mud.
These unassuming-looking waders are tactile feeders. They are able to detect molluscs buried under wet sand from changes in the pressure of water they sense through their bills. Prey are swallowed whole, the hard shells crushed in a muscular stomach so they can be digested.
A place to eat
Life for these shore birds is a race to find enough food to maintain them during winter. The colder the weather, the more energy is used in keeping warm, and the greater the time they must spend eating. However, the intertidal food of the Wash is only available when exposed by the falling tide. As the tide rises again, the knots are forced to retreat up the shore onto a diminishing area of mudflat.
Every fortnight, for a few days immediately after new moon and full moon, the coast experiences spring tides. These are tides that advance and retreat further than usual, affected by the gravity of the moon and the sun. When this occurs, the whole intertidal area is completely covered for up to two hours. The waders are left with nowhere to feed. The result is Snettisham’s knot spectacular.
As the tide rises to reach the uppermost part of the mudflats, the birds become more and more concentrated on the remaining feeding area. Staying until water is swirling around their legs, the knots seemingly defy the rising tide. It is this that gives them their Latin name Calidris canutus, after the 11th century King Canute. According to anecdote, Canute set up his throne on a beach to demonstrate the limits of kingly power. He commanded the incoming tide to halt and not wet his robes. Yet continuing to rise as usual, the tide washed over his feet and legs, forcing him to jump back.
In-built tide table
As the last patches of mud disappear under water, the knot are finally forced to leave the beach. They take to the wing as huge flocks, in a swirling concentration.
“When the tide is only just high enough to cover the mudflats, they wait until the last moment, trying to hang on,” says Jim Scott, the RSPB’s warden at Snettisham. In his 20 years’ experience of the knot spectacle, he has noticed how the birds leave the mudflats earlier on the biggest tides. These are the ones that will more than cover the mudflat. “It is as if they can predict the size of the tides. When tidal coverage of the mudflats is marginal, they aren’t quite sure,” he says. “I have no idea how they do it.”
He recalls one evening when he was alone on the beach. There was no wind, and high tide fell just after sunset. A huge flock of knots rose from the mudflats and flew inland to roost.
“Because the beach was empty of people, they flew just 4ft above the ground. I lay down and watched them as they went over me.” Few people have been privileged to experience thousands of wild birds as close as that.
“I’ve followed my love of wildlife all over the world and seen many amazing species and spectacles,” he says. “Lying down on the grass that beautiful August evening at Snettisham with thousands of knots flying so low over me that I could almost have touched them ranks as one of the best.”
Waiting it out
Having left the mudflats, the knots must roost somewhere while they wait for the tide to turn. Their preferred site is on the steep bank of a lagoon just behind the beach. Here, they pack in shoulder to shoulder with no space between birds.
Waiting here, the knots have no view of the sea. Despite this, they seem to know when the tide has turned, exposing the rich mudflats once more. As if following an unseen signal, they take to the air as one. Together they form a rippling flock so dense no chink of earth or sky can be seen between them.
Jim has seen the birds become impatient and get their timing wrong.
“If they take off early and find the tide still high, they fly round for a while, then have to return,” he says. “If something spooks them, they all go together.”
As quickly as it started, the spectacle is over. Once the knots have crossed the shingle beach, they are back on their mudflats. Now they follow the falling tide down the shore in search of clams. Only when the tide turns will their natural instincts act to save them at the very last minute. And so the performance is repeated once more.
The knots that overwinter in Britain belong to the species Calidris canutus islandica. Approximately 320,000 birds come here, which is approximately one third of the world population. The greatest numbers are found in large muddy estuaries such as the Wash, Morecambe Bay, the Thames, Humber and Dee estuaries, the Solway Firth and Strangford Lough.
Bird ringing studies have shown that these birds breed in Greenland and the Canadian high Arctic, particularly Ellesmere Island. The round trip distance between Ellesmere Island and Norfolk is 5,000 miles. However, they do not fly direct, and some are known to stop off in northern Norway. This adds another 2,800 miles to the journey.
Experiments have shown that migratory wading birds have an in-built compass capable of detecting the earth’s magnetic field, and can also navigate using the stars. This navigational ability is not learned, but inherited. The young birds born in northern Canada find their own way to Snettisham after their parents have left.
At their breeding grounds in Greenland and Canada, knots behave very differently to the great flocks seen in Britain. They become highly territorial, aggressively defending their nesting sites from other pairs.
On arrival at the breeding grounds, their bodies undergo great changes. There is a reduction in flight muscles and fat, coupled with an increase in the size of their digestive and reproductive organs. These latter changes enable them to recover from the long migrations and get into condition for the demands of maintaining and defending territories, laying eggs and rearing young.
The birds nest on the ground in a shallow scrape lined with leaves, lichens and moss.
The female usually lays four eggs, well camouflaged against the Arctic vegetation.
Males and females share incubation duties equally, allowing each parent to spend half the time feeding. The females leave before their young can fly. Once the young have fledged, the male begins his migration south. The young birds make their first journey alone.
Witnessing the Knot Spectacular
Approximately 25,000 people a year come to Snettisham to watch the knot behaviour. The RSPB put up a new timber screen with observation hatches at the southern end of the lagoon in 2015. This gives a good view of the roosting birds and their take-off after high tide, without disturbing them.
The best tides usually occur around sunrise in autumn and winter. It is a walk of approximately
1.5 miles from the RSPB car park to the beach and hides to get the best view. The walk there, or back in the case of an evening tide, is likely to be in semi-darkness so a torch is useful. Tide tables can be found on the RSPB website at www.rspb.org.uk