As the cold weather approaches, airborne species from across the northern hemisphere will take up temporary residence in Britain in the quest to survive. Adrian Thomas reports.
With winter fast approaching, Britain can seem an unwelcoming place for wild birds. The swallows and cuckoos and millions of other birds that visit these shores in the summer months have long since departed, leaving our resident birds to rough it out. But there are millions more birds that actually choose to come here for the winter. It is hard for us central-heating-loving humans to understand why this should be; the thought of spending just one January night outside is enough to make us shiver. And yet it appears that for many species our islands are the "must visit"'
Reasons for attendance
The reasons why they elect to come here, and the means by which they survive once they have arrived, provide a fascinating subject for observation and study.
So which are these madcap species that head here as winter approaches? The birds we instantly know must be winter visitors are those that are completely absent from our shores at other times of year. Take Bewick's swans for example, small wild swans with a black and yellow bill. No Bewick's swans breed in Britain - in summer, you will not find one for love nor money.
Come mid-October, however, at a few special places around the country such as Slimbridge in Gloucestershire or the Ouse Washes in East Anglia, Bewick's swans arrive as if out of nowhere. Their return can be predicted almost to the day and, once in, they settle down and stick the winter out come what may.
Other winter birds whose arrival is made obvious by their total summer absence include several species of wild geese, some of our many wading birds, plus smaller birds such as redwings and fieldfares, both of which are thrushes, and the brambling, a jazzy little finch.
But many winter birds are less obviously visitors as some of their kind also live here year-round. In some cases, resident populations are swollen by massive numbers of incomers. The chaffinch, for example, the familiar garden finch with the pinkish breast and white wing-flashes, has a British breeding population of some seven million pairs or more. Over the winter, these numbers are doubled by the squadrons of foreign chaffinches that arrive in October and fan out across the country.
Similarly, starling and song thrush numbers are hugely boosted, as are those of almost all of our duck species as well as the gulls we see gathering on playing fields and reservoirs. Even in those species whose numbers don't appear to have increased at all, there is often in winter a smattering of newcomers mingled in among the resident birds.
North, east, west... and south
Where are all these birds coming from? Logic suggests that if summer visitors have gone south, then our winter visitors must be from "up north".
But directly north from Britain there are only the small islands of the Faeroes, and then nothing but sea and ice until you reach the North Pole.
There are certainly no huge Faeroese populations of birds to come flooding into Britain.
To the north-west is Iceland. From its midge-infested wetlands come whooper swans and pink-footed geese, winging their way across 800km of open Atlantic. On a good tail-wind the journey can be done in 12 hours, though it often takes much longer. Alongside them comes the merlin - a small dashing falcon - and various wading birds such as black-tailed godwit, golden plover and dunlin.
Beyond Iceland is Greenland, most of it an inhospitable icecap some 3,000m high. In summer, it is warm enough in parts of the coast for some birds to breed. The dainty barnacle goose raises its goslings on Greenland's eastern shores, and the entire population flies via a stopover in Iceland to winter in Ireland and on Islay in Scotland. From Greenland's western seaboard come the red-throated diver and the knot - named after King Canute - which runs along the sea's edge and appears to try, like the ancient king himself, to hold back the tide.
And there are birds that come from even further north and west.
Light-bellied Brent geese and purple sandpipers fly from the Arctic islands of northern Canada, places such as Ellesmere Island and the Queen Elizabeth Islands. The birds share their summer home with the Inuit and polar bears 4,500km distant on a longitude 100 LESS THAN west.
Our winter visitors also come from the east. Spitzbergen, some 3,000km from Britain, is the source of yet more barnacle and Brent geese. Britain is the only place in the world where this population of barnacles spend the winter. To the south-east of Spitzbergen is the northern coast of Siberia.
Here are the nesting haunts of bar-tailed godwits that winter on our estuaries, and of those Bewick's swans. Some birds even come from as far east as the Taymyr peninsula, a distance of some 4,600km.
But the barren, treeless islands and "wastelands" of the north don't explain the massive numbers of perching birds that come here in winter. The obvious source of these is Scandinavia, just a short hop, relatively speaking, across the North Sea. At dawn in late October on our east coast, it is possible to stand and watch streams of finches, starlings and thrushes arriving. Curlews, woodcocks, short-eared owls, grey herons, blackbirds, goldcrests and many other species head here from Sweden and Norway.
Ringing studies have shown that many small birds also come to Britain from much further south and east than Scandinavia. Starlings come from northern Poland, and redwings from Finland and Russian Keralia. Out of a great swathe of eastern Europe and western Russia come wintering ducks such as wigeon and teal. Some may come from as far south as the Caspian Sea or even the Black Sea.
And some birds buck expectations and travel north-east. Over the past few decades, the blackcap, a small woodland bird that was once just a summer visitor from Africa, has taken up winter residence. Our breeding blackcaps still flee south as autumn kicks in but then, in November, others appear in gardens and at bird tables. They breed in the Low Countries and Germany and have successfully adopted a new migratory pattern.
Britain - a migration magnet
The picture emerges of Britain standing out as a magnet towards which birds are being pulled from across half of the northern hemisphere. Several million birds take great risks and expend considerable amounts of energy to come here. Why? Their survival depends on it. The key to it all is temperature. We may find it cold, but that average temperatures here rarely drop below freezing. It is not that birds are unable to withstand freezing conditions - they are, easily. The Arctic redpoll, for example, a small frosty-looking finch with a bright red forehead, has been recorded living in temperatures as low as - 67 LESS THAN C.
It is all to do with "energetics". Birds are warm-blooded creatures - a few degrees warmer than humans - and maintaining that body temperature requires energy. The colder it is, the more calories have to be burned, and hence the more food must be found. Bathed in the Gulf Stream, Britain may be miserably rain-soaked and grey for much of the time, but food does not get covered in a perpetual blanket of snow or locked beneath iced waters or in permanently frozen earth.
Wild Cafe Britain, it seems, is almost always open. And as the closest diner on the migration highway, why travel further?
A year of living dangerously
But that is not to say that surviving the British winter is a doddle for our birds. If conditions turn bad, it can be a time of great hardship. And if the weather is atrocious, the mortality rate can be astonishing. The winter of 1962-63 was the worst for wild birds in the past 100 years, at least. It was the first and only winter of the 20th century to record an average temperature in central England of below zero.
The effect on bird populations was catastrophic. It is possible that almost half of all wild birds in Britain died during the freeze, which lasted from Christmas to March. The worst affected was one of our most beautiful birds, the kingfisher. It feeds almost exclusively on fish, caught by diving into open water. As pools and rivers froze across the country, kingfishers starved and in many areas were wiped out completely.
Birds do at least have some weapons in their armoury to counteract the hazards of winter. Adaptability is crucial. Driven by hunger, many species tuck into unusual meals or are more bold in their search for food. Great spotted woodpeckers are thought to have ventured on to birdtables for the first time in the hard winter of 1947-48 and, in the same year, greylag geese in Scotland turned to feeding on swedes; both species have maintained those habits since. Cold weather has forced song thrushes to feed on periwinkles, little owls to come to bird tables to take fat (and birds), and moorhens to sneak into farmyards to roost with chickens.
Some birds make clever preparations for winter by filling the larder in advance. The best exponent of this in Britain is the jay. Usually a shy skulker of woodland, this peachy-coloured relative of the crow can frequently be seen in autumn flapping through town and countryside on its big rounded wings. Its mission is to seek oak trees from which to harvest acorns. They cram up to 10 at a time into their crop and then fly off to bury their bounty, memorising the location of as many as possible for reclamation later.
Preparation can be physiological, too. Many birds develop a thick undergarment of down. These specially modified feathers do not hook together in the neat way that contour feathers do, but can be fluffed into a myriad of air pockets that insulate the bird. Their effectiveness is well known from our use of the down of the eider duck.
Birds also use autumn as a period for thickening the insulating layer of fat underneath the skin. Surprisingly, though, few birds turn to each other to share body warmth. Even birds that roost close together often maintain some "personal space" around themselves. In just a few species, group huddles are essential for survival. The tiny long-tailed tit roosts in family parties, the whole troupe lined along a branch, the dominant birds snuggled in the middle of the row.
Most birds, even if they don't like body contact, take great care in their choice of roosting site. Tucked away in dense thorn bushes, tangles of ivy or bramble, or deep in a tree hole or rock cavity, they avoid the worst of the rain and wind. Some birds, such as coal tits, even disappear down mouse holes in the ground.
In all but the most sheltered roost sites (lofts, for example), heat loss is a problem, especially from the exposed parts of the body, such as the legs and beak. Many birds compensate by tucking their head under their back feathers, or in some case by standing on one leg.
Water birds have a nifty adaptation by which warm blood flowing into the top of their legs passes close by vessels bringing chilled blood back from the feet. The heat is transferred so that it is cold blood, sometimes barely above freezing, that permanently circulates around the lower leg.
If weather conditions turn really bad, birds do have the option of moving, far and fast. Cold weather movements can often be observed as straggly lines of lapwings and chirruping skylarks beat a retreat, many birds crossing over to Ireland or the western French seaboard.
Multiplication's big pluses
If all these techniques fail, species which suffer catastrophic losses in hard weather often bounce back, . A pair of kingfishers, for example, can, in a good breeding season, produce four broods of seven eggs each, and this year's chicks can breed in the following year. As long as harsh winters don't fall back-to-back, numbers can quickly recover. When average winter temperatures shift, we inevitably see bird behaviour changing to suit. The recent run of mild winters, whether or not the product of human-induced climate change, has given us an insight into what a globally warmed future might hold. Little egrets now have no problem finding fish around the English and Welsh coasts, Dartford warblers on English heaths have reached population levels never recorded before, and robin numbers are up to 25 per cent higher than 30 years ago.
Conversely, some birds may not need to travel all the way to Britain in future, stopping instead at Netherlands or Denmark, and birds such as Bewick's swans may eventually disappear from our winter landscape.
Given their survival struggles, it has become something of a national winter pastime to feed our garden birds (for more information, see box on previous page), and there is no doubt that this has increased the survival rate of many.
* There are plenty of websites that will allow you to calculate the shortest surface distance between two points on the Earth's surface. This will be an arc of a circle whose centre is the centre of the Earth, and is known as a Great Circle. Some birds are thought to migrate following Great Circles, despite this requiring constant recalculations of bearing.