As predators at the top of the freshwater food chain, herons may be excellent indicators of environmental health in the countryside. They build large stick nests, mostly in colonies at traditional sites, thus lending themselves to direct counts of active nests.

The BTO Heronries Census began in 1928 and is the longest-running breeding-season bird monitoring scheme in the world. The aim of this census is to collect annual nest counts of Grey Herons from as many sites as possible in the United Kingdom. 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, provide a clear measure of the population trend.

In recent seasons, observers have also counted the nests of Little Egrets Egretta garzetta, which have been appearing in an increasing number of heronries since the first UK breeding records in 1996 and are now nesting as far north as Cumbria, and even of Cattle Egrets Bubulcus ibis, Night-herons Nycticorax nycticorax and Spoonbills Platalea leucorodia. Since egrets are fully included in the Heronries Census, data are requested from all breeding sites, whether or not Grey Herons are also present. Data submitted for the Heronries Census for Little Egrets and other rare species are shared with the Rare Breeding Birds Panel, who hold the more complete data sets. Counts of Cormorant colonies, which often occur alongside heronries, are also recorded and contribute to broader monitoring of that species (Newson et al. 2007, 2013).

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. The number of heronries counted each year has grown in recent years to more than 600. Around two-thirds of the heronries in England and Wales are currently counted each year, with more-complete censuses carried out in 1929, 1954, 1964, 1985, 2003 and 2018. Historically rather few counts have been made of heronries in Scotland and Northern Ireland, except during the special surveys, but support there for the Heronries Census has been growing fast in recent years. Almost all the known heronries have been counted in Northern Ireland annually in recent years.

Online data submission was made available for Heronries Census observers for the first time in 2015.

Data analysis

Population changes are estimated using a ratio-estimators approach derived from that described by Thomas (1993). Essentially, the ratios of the populations in any two (not necessarily consecutive) years of the survey are estimated from counts at sites visited in each of those years. These ratios can be used to estimate the counts at sites that were not visited, and hence build an estimate of the total population. The population model also allows for cases where the extinction of colonies and the establishment of new ones had not been observed directly (Marchant et al. 2004).

Data presentation

On the Grey Heron page of this report, the UK trend is presented graphically as annual estimates of apparently occupied nests, with a smoothed trend and its 85% confidence limits. The smooth trend line is based on a non-parametric regression model, using thin-plate smoothing splines with degrees of freedom approximately 0.3 times the number of years in the model. Trends are also shown for England and Wales together, and for England, Wales and Scotland alone.

Visit the Heronries Census page of the BTO website.