I don't pretend to understand. It seemed so simple in the past. Then, a species of bird was a group of birds capable of interbreeding and producing fertile offspring. The boundary between species seemed clear-cut. Different species of birds could hybridise, but their offspring would be unable to produce young. I thought 'Elsie' (the Lesser Crested Tern that summered on the Farne Islands in the 1980s/90s) was therefore doomed to fail despite often mating with Sandwich Terns, laying eggs and raising the occasional chick. That's what I presumed about the difference between species, but it seems this no longer holds true. Across the northern hemisphere the Herring Gull has developed a number of distinct, self-contained populations, and each varies slightly in terms of combinations of characteristics that include plumage, vocalisation, flight patterns, behaviour and even structure. The Herring Gull has been split into 5 species and these include 'Caspian Gull'.

Some birdwatchers get very excited about 'splits' and the new 'species' available to tick and adding to personal lists, but I am not yet one of them. So I had decidedly mixed views about twitching the adult Caspian Gull that arrived in Amble this month. Yet I went and added both the 367th species to my UK list and the 326th to the list I most care about - my one for Northumberland. But can I really tell the difference? I concede this individual looks longer-billed, longer-winged and longer-legged than a typical Herring Gull. Yes, the head shape seems different too - flatter forehead and more angular crown. And, yes, the colour pattern on the primary feathers looks unusual with longer white 'tongues' extending into the black wing tips. But the call? More 'childlike' or more 'throaty' as it has been described to me? I cannot pretend to hear the difference or to know. And I feel compelled to ask. What is a 'typical' Herring Gull?

Mark Winter

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