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Arctic Tern (Sterna paradisaea)

Arctic Tern (Sterna paradisaea)

Arctic Tern, Northumberland, UK (Dave Kjaer)

No species of bird on earth performs a longer migratory journey than the Arctic Tern. From its breeding grounds in northern Europe it coolly transfers to the waters of the Antarctic for the winter, travelling at least 17,500 km in the process. This journey takes it down the coast of West Africa, to Southern Africa and beyond, and once the birds have reached the supremely rich fishing grounds of polar waters, they meander among the icebergs, spreading out west or east, and occasionally, almost incidentally, make a complete circuit of the Antarctic Continent. Having fed among the Penguins for a few months they return north, using much the same route – except, that is, in the latter stages, where evidence from birds recovered in the Ural Mountains suggests that some may take an extensive overland route, probably at great altitude. So they return to their colonies having had many adventures and, taking into account their Antarctic jaunts, travelling as much as 50,000 km in a year. And in that same year they will have seen more hours of daylight than any other animal on earth, especially those that breed above the Arctic Circle.

Despite being exposed to so much daylight, and with it so much opportunity for feeding, the Arctic Tern adheres to quite regular foraging times, usually being most active in the early morning and evening. It has a fairly varied diet, with crustaceans and insects being consumed along with the usual fish. In recent years the bird has expanded its range in the Baltic Sea by feeding on midges, which themselves are increasing because of silting up. In these areas it is trespassing into territories previously occupied only by the Common Tern. When diving for fish, Arctic Terns are characteristically more hesitant than Common Terns, often hovering several times at different heights before literally taking the plunge.

Arctic Terns, Northumberland, UK (Dave Kjaer)

The breeding behaviour of Common and Arctic Terns is so similar that it seems remarkable that the two species hardly ever hybridise. Admittedly, the courtship of the Arctic Tern has more of an aerial element to it, with the females, for example, typically taking flight as soon as they are approached by a male. But perhaps the nesting sites are a more important factor. Arctic Terns usually nest on more barren beaches or islands than Arctic Terns and, where the two mix, the Common Terns take the grass and the Arctic Terns the bare rock (they have shorter legs). There is a climatic difference, too, with the Arctic Tern reaching into far more extreme latitudes than the Common – indeed, 60% of the breeding population is found in Iceland.

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