This study documents the fourth survey (198 1–1982) of bridled guillemots and provides the most complete information yet available on the distribution of the bridled form of the Common guillemot. The distribution has remained largely unchanged since the last survey (1959-60), although marked changes have occurred at a few colonies. Sea surface temperature “explains” only 35% of the variance in the proportion of bridled guillemots. Minimum air temperatures during the breeding season are much more closely correlated with bridling and “explain” 60% of the variance. No relationship exists between sea temperatures and the proportion of bridled guillemots in wintering areas. The distribution of bridled guillemots may represent the slow, southward spread of the bridled form. Alternatively, selection may favour the bridled form in cooler areas. There is insufficient evidence to distinguish between these two hypotheses, but if selection is important, the correlations with environmental factors suggests that it operates during the breeding season rather than at other times.
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