Unravelling the Chaos in the Ledges
Abstract from Ibis
7th September 1996
Anyone who has seen a Guillemot colony in full swing will realise just how important the nest site is for the breeding birds. Their upright stance, even when incubating, means they are capable of nesting at a higher density than any other seabird.
Mike Harris, Sarah Wanless and Tim Barton have different parts of the dense colonies on the Isle of May, in the Firth of Forth, under intense observation for much of the breeding season. Their birds are individually colour marked and the results of their logging of territorial birds appear in the most recent issue of Ibis (138, 399-404).
They followed the sites of 470 birds with at least one year of breeding experience and found that the average was 85.7% of the birds were on exactly the same territory in the two years. For the next year it was 91%. Of the 14.7% of birds that did not return to the same site 40% (almost 6% of the total) were not seen. These might have died or moved to a part of the colony which could not be checked - even to another colony. A further 35% (just over 5%) had moved breeding site - mostly by less than two metres! And the rest (25% - 3.7% of the total) had a non-breeding year.
The situation seems to be very complex but one suggestion they have is that the minority of birds who changed nest site and improved their breeding success chose to move. The ones which were less successful were probably evicted.