As top predators in the freshwater food chain, herons are able to thrive only when their potential prey are also present in good numbers. Monitoring their breeding population sizes can therefore provide a valuable indicator of the health of the freshwater environment.
Counting nests provides by far the most efficient and accurate measure of breeding numbers for most colonial birds. For this reason, BTO monitors colonial herons through special surveys that collate counts of 'apparently occupied nests' at colonies.
The BTO Heronries Census collects nest counts of herons from as many heronries as possible in the UK each year. The main species covered is Grey Heron but Little Egret is fully included, as are rarer species of colonial herons such as Cattle Egrets (which nested in the UK for the first time in 2008). Nest counts of Cormorants are also collected, especially where they are nesting alongside herons. Data are shared with county recorders and for rare species with the Rare Breeding Birds Panel.
The long history of the Heronries Census has been interspersed with periodic Heronries Surveys which have expanded upon its coverage in particular years. Heronries Surveys (of varying aims and scope) were undertaken in 1928, 1954, 1964, 1985 and 2003. The count data from all these surveys are fully subsumed into the Heronries Census database.
A further survey that observers can sometimes carry out at a heronry is to compile a colony Nest Record Card. This involves inspection of nest contents and is part of the Nest Record Scheme rather than the Heronries Census.