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Great northern diver. The words themselves conjure up an image of somewhere vast, very cold and very wet! True to its name, this species breeds on the edges of lakes in the treeless expanses of the subarctic, in areas like Iceland and Greenland. As you would expect, all the freshwater in these areas, not to mention large areas of sea, are frozen solid by December. In order to be able to continue hunting for its favoured prey, medium-sized fish, great northern divers move south, usually to marine waters. This gives coastal birdwatchers in Britain and Ireland the chance to find this impressive species and the shows just how good this winter has proved to be for seeing them.

Cold and stormy weather also pushes some birds onto inland lakes and reservoirs. The Bird Track map below, showing 10km squares in which great northern divers were reported, indicates at least seven inland locations where this unusual bird has been seen recently. Last week Grafham Water in Cambridgeshire hosted six individuals, an unusually high number for an inland site.

At nearly 1m in length and with a wingspan closer to 1.5m, the great northern diver is the largest species of diver regularly recorded in Britain and Ireland. Only its close relative the yellow-billed diver is bigger but they are very rare visitors to our waters.

Divers go by a different name across the pond: loons! There are two explanations for this name. The first is their movements out of water; the feet are positioned so far back on the body that they appear to be lame (from the old Norse word: l艒mr). The other explanation is that they were named with the Middle English word for a crazy person, 'loun', on account of their haunting, wailing breeding calls.

And finally, if you thought snorkelling was something reserved for warm-water beach holidays, divers make a habit of it all year round, even in the waters around our coasts. Prior to fully submerging, divers will often swim around with their heads under water, checking to see if there are any juicy fish worth chasing. Come to think of it, I think I would do the same rather than plunging into the icy depths on the off-chance!