Distribution of the Bridled Form of the Common Guillemot (Uria aalge)

Abstract

THIS subject forms an excellent illustration of how character gradients in and between geographical races of animals submit to the treatment suggested by Huxley¹. In this instance, the cline—to make use of his term—is one in the proportions of polymorphs. The bridled form of Uria aalge, a mutant, which apparently depends on a single gene difference and is distinguished by a white circle around the eye continuing backwards as a line towards the region of the nape, represents a larger percentage of the guillemot population in the north of its range than in the south. It is clear that more details of this cline will reveal important facts, such as the groups that are situated on it and the relationships between them. During the past breeding season, the British Trust for Ornithology has organized a cooperative inquiry into the status of the bridled guillemot in the British Isles, and, although it is hoped to continue this inquiry during the coming season to fill in the remaining gaps, enough is now known to encourage a preliminary publication.