Animal

Weatherwatch: how a Siberian sprite 'reverse migrates' to Britain


The smallest British bird is the goldcrest, along with its much scarcer cousin, the firecrest. Both are just 9cm long and weigh 5-6 grams – roughly the same as a single sheet of A4 paper or a 20p coin. Yet one species of warbler is just as small, though a gram or so heavier. Despite its size, Pallas’s warbler – named after an eighteenth-century Prussian ornithologist – travels all the way from its Siberian breeding grounds to Britain.

Until the 1980s this Siberian sprite was a very rare vagrant, with only a few occurring each autumn. All that changed in October 1982, with a record influx of at least 120 Pallas’s warblers, mainly along the east coast. Six years later, in October 1988, two more influxes occurred, turning the species from a major rarity into a regular arrival.

But given that Pallas’s warblers usually spend the winter in south-east Asia, why are they coming to Britain at all? It appears that some are heading in more or less the opposite direction from what would be expected, a phenomenon known as “reverse migration”. Many perish en route, but if the weather conditions are right – an anticyclone over Scandinavia bringing easterly winds across the North Sea – they end up on the UK’s shores.



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