Friday, 4 January 2013

Gulled by an aberrant gull

Leucistic Herring Gull
One technique used in the study of wild birds is ringing. Usually a small, numbered metal ring is placed round the bird’s leg, so that if it is later recaptured, or found dead, the ring may help provide information about migration, longevity and other aspects of its life. 

Some birds may also be fitted with brightly coloured plastic leg rings that allow them to be identified without recapture and therefore with minimum disturbance. Each bird is given a unique combination of small rings of different colours, which are read in a specific order. 

Larger birds may instead be fitted with rings bearing conspicuous letters or numbers that can be read in the wild with the aid of a telescope. I recently had an encounter with one such bird that has become something of a celebrity among London birders. 

I was sitting in a hide at a reservoir in north-west London when another birder thought he had spotted a rare Iceland Gull. But the bird soon turned out to be an aberrant example of a Herring Gull — a common species. The bird had extremely pale (leucistic) plumage, with all-white wings (like an Iceland Gull’s) rather than the Herring Gull’s usual grey wings with black and white tips. 

The four-character alpha-numeric code on its leg rings confirmed its celebrity status. The bird had been ringed a few years earlier by members of the the North Thames Gull Group, who use a cannon net to snare birds feeding on waste at landfill sites on the Essex side of the Thames estuary. Once caught, the gulls are extracted from the net and marked with numbered leg rings. Some, like the bird in question, are also given orange rings bearing large letters or numbers that can be read with a telescope. 

So is this gull’s celebrity due to its leucistic plumage? Not entirely. Its birding fame derives mainly from the characters on its four leg rings — S, H, 1 and T. These more or less spell out an expletive that birders may use when they realise that the pallid gull is not after all a rare species.

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