Tuesday, 9 October 2012

Guillemot population - Skomer 2012

Guillemot population this year reached the dizzy heights of 22508 indivduals! The population must be reproducing enough young or recruiting birds from other colonies to match this expansion. Either way it is a sign that Guillemots want to be on Skomer. The study plots* have not quite showed the same level of expansion over the past few years which could suggest that these study plots are "full" or the whole island counts are becoming less accurate as the numbers of birds increase (e.g. having to count groups of 10 or 50 rather than individuals). Once again it shows that the population is definitely not in decline.
Guillemot population on Skomer 1963-2012 (Source Skomer Seabird Report 2012, unplub.)

Trend in UK abundance index (solid line) of common guillemot 1986-2011 with 95% confidence limits (dotted lines).Based on SMP data (Source JNCC: Guillemot Status and Trends: Accessed here http://jncc.defra.gov.uk/page-2898)

A typical Guillemot Colony on Skomer - The Wick

Productivity was 0.63 (1989 - 2012 Mean = 0.69), higher than last year but showing a downward trend. This lower productivity could be due to the fact that there are more guillemots in the study plots, some of which will be non-breeders but might be registering as "breeders" in our data. Work by other universities on the islands suggests that productivity is constant. If we are having a lower productivity this is demonstrating a "density-dependance" on Skomer whereby we might be reaching our natural peak of guillemot population - after several years of lower productivity there may well be a reduction of the population.
Guillemot productivity on Skomer 1963-2012 (Source Skomer Seabird Report 2012, unplub.)

Chris Taylor
Skomer Warden

* (study plots are smaller areas counted up to 10 times where as the whole island counts we aim to count each section of cliff two or three times only)

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