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Within the Common guillemot Uria aalge colony, at the Gannet Clusters, Labrador, Canada, there was no evidence of heterogeneity in the proportion of bridled birds in different parts of the colony. Nor was there any evidence for assortative mating among bridled guillemots. Examination of the distribution of bridled birds in rows of guillemots however, showed a significant degree of clumping. Two hypotheses are presented to explain this distribution pattern: (a) the environmental hypothesis, which proposes that bridled birds preferentially select particular micro-habitats, or choose to breed where there are already bridled birds. (b) the return hypothesis, which proposes that one sex (probably female) returns to breed very close to where it was reared.