We think there may be several contributory factors to this, including improvements in water quality, a reduction in persecution and an increase in suitable nesting and feeding sites as gravel pits have been flooded and restored.
Overlain on the increase are dips in population caused by exceptionally cold winters, especially those of 1946/47 and 1962/63. Each dip has been followed by recovery within a few seasons.
Whilst the Census aims to cover all heronries annually, more focused surveys are run periodically with increased promotion to make an effort to increase coverage. The most recent such surveys were in 2018 and 2003.
Further analyses are ongoing.
Heronries Survey 2003 occurred when population levels peaked and therefore, although fewer sites were covered than in 2018, it provided the highest count of active nests ever recorded in any one year, with 10,411 apparently occupied Grey Heron nests counted in the UK (out of an estimated total of 13,797). The map shows the active heronries known that year. There were none in Shetland or the Channel Islands. The Republic of Ireland was not covered (although a few counts were received).
There was a concentration of large heronries in Cheshire and Greater Manchester and another in the London area. Estuaries and large lakes typically hold large heronries. Exposed coasts, especially west-facing, are generally avoided. Upland regions are occupied but heronries there tend to be relatively small.
Read the 2019 Heronries Census Summary
The most recent Heronries Census estimate of UK Grey Heron population size is 9,940 apparently occupied nests in 2019.
Working together for seabirds
BTO work supports effective monitoring of our seabirds and aims to provide opportunities for a new generation of seabird surveyors.