Unlike their familiar resident cousin, the mute, these long-distance migrants - often referred to as "wild" swans by ornithologists - choose to split their year between the vast wastes of the Arctic and waterlogged British farmland. Whoopers are the larger, and though they are about the same size as a mute swan, they swim with a much more upright neck and head and have yellow, rather than orange, bills. Also, unlike mutes they never raise their wings above their backs and, when in flight, they make no "singing" noise: simply a rhythmic swishing.
Thanks to their yellow bills, at a distance a group of Bewicks can easily be mistaken for whoopers but, in reality, they are much smaller. The simplest way to distinguish them is by their calls (whoopers "bugle", while Bewicks call like geese). Also, they have shorter necks which they are more inclined to arch. From now until spring both will be a familiar sight at a handful of our best wetland reserves, where they will remain until lengthening days draw them back to the frozen wastes.
At first glance this may seem like a curious way to divide your year. Why would any creature - let alone a bird with all the problems of keeping eggs warm - opt to breed in the chilly Arctic summer? In fact, the answers are relatively simple.
The great attraction of a summer near the poles is linked to daylight. While the tropics may bask in a seasonless year of steady temperatures, days are always about 12 hours long. The further north or south you go, however, the more elastic daylight becomes. The result is that animals breeding in these regions have far more time to search for food.
A good demonstration of this is the peregrine falcon which, as a global species, makes direct comparisons easy. Pairs breeding in the tropics typically raise only one youngster a year; British residents average two or three; while the migratory tundra subspecies can manage up to five. In the case of the Bewick and whooper swans this tactic is also highly successful. They are among the few true migrants capable of making long- distance flights to such remote areas. Also, as large birds they have the body weight to sustain the frequent cold snaps that occur even in midsummer. This means that for six months they cash in on the many tundra pools erupting with insect and other invertebrate life and the explosion of hardy grasses and flowers, making the most of 22-hour days. All this acts as the natural equivalent of a giant free buffet for the swans.
There are other appeals, too. As large birds they are relatively safe from most predators. Apart from a handful of Arctic foxes and skuas, they face few risks on their tundra breeding-grounds. All this results in breeding pairs - which mate for life - raising four or five youngsters.
When, in October, the first snows force them to look for pastures new, the family flies south as a group. It continues to stick together throughout the winter, splitting up only in spring when the youngsters reach sexual maturity, although most don't breed until the following year. Normally the annual arrival of the swans would be almost finished by now, but this year is unusual.
Whoopers have arrived in force somewhat earlier than normal, but the reverse is true for Bewicks. Normally the Wildfowl and Wetland Trust (WWT) would expect at least 1,000 birds at its Welney reserve, while at its Slimbridge sanctuary there are only about a dozen birds - well down on the normal 60. This disruption is due entirely to the weather, which has been dominated by westerly gales. These strong headwinds have seriously hampered the influx of Bewicks. In addition, thanks to flooding, Dutch farmers have lost much of their potato crop, which lies rotting in semi- submerged fields. Root crops are an important part of the Bewick's winter fare and the flocks that should arrive here any day have been delayed, feasting on the unexpected bonanza on the other side of the Channel.
Conversely, however, the winds have helped the whoopers. Normally Britain gets few whoopers because it is at the southern extremity of their range, but because of the strong tailwinds this year, their two prime wintering sites, Martin Mere in Suffolk and Welney in Norfolk, are now close to reserve records, with roughly 1,000 birds at each.
For the next four months, there is ample opportunity to watch our biggest and most spectacular winter migrants feeding on the flood plains of the East Coast and Severn Estuary.
Visit the WWT's Slimbridge Reserve near Gloucester (01453 890333) or Welney on the Wash (01353 860711). There are floodlit swan feeds Sat and Sun evenings all winter and the Trust also runs a pounds 25 sponsorship scheme of ringed birds