The aim of this census is to collect annual nest counts of Grey Herons Ardea cinerea from as many sites as possible in the United Kingdom. The Heronries Census began in 1928 and is the longest-running breeding season monitoring scheme in the world. Volunteer observers make counts of apparently occupied nests at heron colonies each year. Changes in the numbers of nests, especially over periods of several years, are a clear measure of the population's trend. In recent seasons, observers have counted also the nests of Little Egrets Egretta garzetta , which are now appearing in a number of southern English heronries.
Coverage is coordinated through a network of regional organisers.
A core of birdwatchers and ringers monitor their local colonies annually, providing a backbone of regular counts. Rather few counts are made of heronries in Scotland and Northern Ireland. Counts are submitted to the BTO on cards and the data are entered onto computer at BTO headquarters. The number of heronries cards submitted each year is around 450. These ratios can be used to estimate the counts at sites that were not visited, and hence an estimate of the total population.