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Increase in frequency of the bridled morph of the Common Guillemot Uria aalge on the Isle of May, 1946-2000: a return to former levels?

Harris, M. P.; Rothery, P.; Wanless, S.. 2003 Increase in frequency of the bridled morph of the Common Guillemot Uria aalge on the Isle of May, 1946-2000: a return to former levels? Ibis, 145 (1). 22-29. 10.1046/j.1474-919X.2003.00118.x

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Abstract/Summary

Bridling in the Common Guillemot Uria aalge is a classic example of a stable ratio-cline polymorphism. Between 1946 and 2000 the frequency of the bridled morph of the Common Guillemot colony on the Isle of May, south-east Scotland, increased significantly from 3.5% to 5.9%. Demographic data collected between 1982 and 2000 indicated that the average breeding success of a pair including at least one bridled bird was 83.8%, significantly greater than 79.5% in unbridled pairs. Over the same period the average overwinter survival was 96.3% for bridled birds and 94.7% for unbridled birds, but the difference was not statistically significant. A population model showed that the increase in frequency of bridling could be accounted for as a return to an equilibrium level close to the 5.04% observed in 1936. Differences in breeding success contributed relatively little to the increase, which could be explained without recourse to differential selection pressure. The latitudinal variation in bridling suggests that bridled morphs are more cold tolerant than unbridled ones. However, the increase in the frequency of bridled birds on the Isle of May was not associated with any decrease in sea or air temperatures over the period.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1046/j.1474-919X.2003.00118.x
Programmes: CEH Programmes pre-2009 publications > Biodiversity
CEH Sections: _ Biodiversity & Population Processes
ISSN: 0019-1019
Additional Keywords: Guillemot, Uria aalge, Isle of May
NORA Subject Terms: Zoology
Date made live: 09 Dec 2008 19:52
URI: http://nora.nerc.ac.uk/id/eprint/5172

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