Return to the Isle of Puffins
The Natural History of a Population of Guillemots (Uria aalge Pont.)
Southern, H.N., Carrick, R. & Potter, W.G. (1965) J. Anim. Ecol. 34(3):649-665
1. The common guillemot, Uria aalge, a pelagic auk, which breeds in colonies on seacliffs around the North Atlantic, has a variant form with a white eye-ring (the 'bridled guillemot') which varies in occurrence from 1% in the south to >50% in the north of its range. Its expression is probably controlled by a single Mendelian factor.

2. Surveys of the proportion of bridled birds over the breeding range in 1938-39, 1948-50 and 1959-60 have shown that it may rise and fall (or vice versa) over the 20 years of investigation over a range of up to 10%.

3. An intensive study by colour ringing at a colony of guillemots near Aberdeen has shown that (a) the survival rate of adults is high (of the order of 0.87 per annum) and (b) birds are faithful to their sub-colony and move little, if at all, to other sub-colonies. It is likely that any mixture between sub-colonies is due to young birds coming to the cliffs to breed for the first time in their second or, more probably, their third year of life.

4. Since the 'turnover' of the population investigated was relatively slow (the mean expectation of life of adults was 7 years and the generation time 9 years) and movement is slight, the rate of change in ratio of bridled birds recorded during the decennial surveys is incompatible with its being caused by selection.

5. Other possible explanations of these changes in gene ratio are (a) that some of the colonies may be patchy in the distribution of the bridled birds, so that counts to show their proportion may have been biased and (b) that the results of the intensive study reported may not be wholly representative. Other writers have detailed remarkable changes in numbers of guillemot colonies and it is possible that mass movements sometimes take place. Future surveys should be designed to show whether this does in fact occur.

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