The Gannetry of St Kilda

1. Three estimates of the St Kilda gannetry have been made since 1930: 21 300 adults in 1931, 16 900 pairs in 1939 and 17 035 pairs in 1949. This paper brings together the experience of nine visits to the gannetry by the author between 1955 and 1960, and the results of an exhaustive photographic survey in mid-May 1959. 2. The 1959 census was done by counting from photographs taken from air, land and sea: thirty-five different air photographs were obtained on 13 May; land photographs were taken on Boreray from salient points on the ridge; the coastline of Boreray, Stac Lee and Stac an Armin was photographed from the sea. 3. The gannetry was divided into 116 sections which were sub-divided into 672 sub-sections, each of which were counted four times. Special counts were made to assess differences between different types of photograph and the numbers of pairs and single birds present in each type of photograph. 4. A total index of 47 189 birds is converted by separate factors for aerial and land photographs to 44 526 pairs. 5. If the index obtained from aerial photographs is taken as 1, that obtained from land photographs is 1.32 and that from sea photographs 0.52. 6. Evidence that the gannetry is larger than it was in 1939, 1941 and 1947 comes from a comparison of the data for one locality 1939-59, and of aerial photographs of Stac an Armin in August 1941, and Stac Lee in July 1947 and May 1959.