Grey HeronsArdea cinereabreeding in Scotland: numbers, distribution, and census techniques

Open Access
Publisher Website
Google Scholar
Abstract
Between 1976 and 1985 there were records of 349 different heronry sites in Scotland. In 1985, BTO observers located 221 of these heronries and counted 1635 occupied nests. Intensive work at 20 colonies showed that, on average, BTO observer-counts represented about 68% of the total number of pairs of Grey Herons breeding at a colony. On this basis, about 3 800 pairs of herons are thought to have bred in Scotland in 1985, but this figure is of unknown accuracy. The commonest heronry size was 'one' nest; colonies of more than 25 pairs were rare and associated with extensive areas of heron habitat. Breeding density varied from less than 0.3 pairs per 100 km2 in inland Caithness and Sutherland to over 15 pairs per 100 km2 in North Uist and Benbecula. Only about 17% of the Scottish population was breeding in upland environments, whereas 57",% bred near the coast. There is evidence of historical changes in the distribution of breeding Herons in Scotland as the species expanded its range in the north and west prior to 1940, becoming less aggregated as it did so. These changes do not appear to have occurred elsewhere in Scotland, or in the north and west since about 1940. A population index for herons breeding in two separate parts of Scotland is arbitrarily set at 100 for 1985. Earlier censuses, and annual counts from 1981–86, are indexed with confidence limits. The indices for the north and west show no significant variation whereas those for elsewhere in Scotland show a significant low in 1964 and peak in 1981. The small samples and high variability of counts in past censuses have meant that deviations in population of less than 30% could not be detected. Much variability between consecutive counts can be reduced by using the same observer from year to year but it seems unlikely that enough heronries can be counted to detect a decline of less than 10% in the heron breeding population.