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The Common gdlemot, or murre (Uria aalge) is a cliff-breeding auk distributed over the northern Atlantic and Pacific Oceans. In the Atlantic population a mutant form with a white eye-ring, called the bridled guillemot, occurs in a low proportion (1 to 6 per cent) at Englieh and Irish colonies and this proportion increases northwards to a maximum of c.60 per cent in Iceland and Bear Island. This pattern of distribution WBB established by a co-operative inquiry sponsored by the British Trust for Ornithology in 1938–39.

Ten yem later the survey waa repeated and at a Substantial number of colonies the percentage of bridled birds had changed significantly, mainly in the direction of decrease. A recent re-survey after another ten years (1969–60) show that them changes have been almost exeotly reversed and the detailed evidence is presented in this paper.

Additional data from colonies in Norway and in Newfoundland and Labrador make the picture of distribution more complete.