Thursday, 23 October 2014

Doomed Sibes

Every autumn UK twitchers begin to twitch with excitement at the prospect of catching up with rare vagrant songbirds that turn up in small numbers in western Europe after leaving their breeding grounds in the temperate regions of eastern Europe and Asia. Many of these small passerines are known among birders as “Sibes” because their main summer haunt is in Siberia. 

These drifters are almost always juveniles on their first migration — birds that have somehow got their navigational knickers in a twist. Many should have migrated south-east to spend the winter in tropical regions of Asia, but they reach north-west Europe because they have a misaligned internal compass and have set off in the opposite direction. 

These vagrants may appear anywhere in the British Isles, but they are most commonly found on islands at the edge of the Atlantic, such as Fair Isle and the many islands of Shetland and Orkney, where they stop to build up their reserves before attempting to continue their misguided journey over the vast expanse of the North Atlantic. 

Many of these birds are identified after being trapped at bird observatories, such as the famous establishment on Fair Isle, or by bird ringing groups on other islands (or, indeed, anywhere on the British mainland). 

The main purpose of these ringing schemes is to check the movements of individual birds and to discover how long they live. But the chance of learning anything from UK-ringed Sibes is remote because, after their brief break for R&R, most of them will continue doggedly in the wrong direction, setting off across the Atlantic until exhaustion causes them to flop into the waves and either drown or succumb to a pelagic predator. 

We know this is their fate because researchers on the Faroe Island have attached tiny radio transmitters to vagrants such as Yellow-browed Warblers and Barred Warblers and tracked their direction of departure once they have refuelled (see here).

Here’s a recent example of a doomed Sibe. A first-winter female Siberian Thrush — an extremely rare vagrant to western Europe — was trapped and ringed at Hus√ły in Norway on 24 September 2014. Three weeks later it was recaptured at Scousburgh on Shetland Mainland, 600 miles to the south-west. Perhaps it had followed the Norwegian coast and then turned west when the coastline veered off in a more easterly direction. After its Scousburgh recapture at dusk on 15 October 2014 the thrush was released but was not seen again. Presumably, like other disorientated Sibes, it carried on over the Atlantic until exhaustion led to its watery denouement somewhere in the vastness of the ocean. 

Knowing all this, I cannot get too excited by the annual appearance of Sibes in western Europe. Instead, as a birder rather than a twitcher, I find it dispiriting that these disorientated young birds are almost certainly heading for an early death.

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