Timing of breeding and estimated fledging dates of gannets at St Kilda in 2012

Information on the timing of breeding, and hence the period when chicks are present in the colony, has rarely been collected for Gannets at St Kilda. However, given the growing pressure for greater access to islands and stacks in the St Kilda group, the National Trust for Scotland, the owners and managers of the archipelago, have identified the collection of phenological data on Gannet breeding as being of high priority. In July 2012, NTS commissioned a photographic survey of Boreray, Stac Li and Stac an Armin from the sea, and a land survey of Boreray only, in order to obtain comprehensive data on the age distribution of chicks and thus the likely schedule of fledging dates. Projections based on a fledging period of 90 days indicated that the peak of fledging on St Kilda in 2012 was 16–22 September. Twelve, 11 and 20% of nests on Boreray, Stac Li and Stac an Armin respectively were classified as still having eggs at the time of the check. About one half of these eggs were likely to be infertile and, assuming that the remainder hatched immediately after the check, 4, 5 and 12% of breeding sites in the three colonies would still have chicks present on 17 October.