Grey herons (Ardea cinerea) are easily recognised and appreciated by most Londoners and capture public imagination. However, despite their characteristic dark wings and black crest they are often mistakenly known as ‘stork’ or ‘crane’.

Herons are at the top of the food chain in freshwater habitats and their presence can indicate a healthy freshwater environment. Huge improvements in the water quality of the River Thames and its tributaries have provided a plentiful food source for herons, allowing them to increase and spread. London is a heron hotspot and in 1985 contributed about 1% of the United Kingdom population.

Grey herons breed colonially, usually at the tops of tall trees on islands that offer some degree of protection from disturbance. All London’s heronries are in such locations, with the exception of Lonsdale Road Reservoir where they nest on tern rafts.

The British Trust for Ornithology (BTO) Heronries Census provides excellent information on heron numbers, which are currently at their highest levels. During the last nationwide census in 1985, London had three heronries containing 114 nests. By 2000, this had gone up to 16 heronries with 302 nests. Heronries are found in the following London Boroughs: Bexley; Bromley; Hillingdon; Hounslow; Merton; Richmond; Waltham Forest; Wandsworth and City of Westminster (see Annex for map and further details). There are also two heronries (Kempton Park and Stocker’s Lake) just outside the Greater London boundary.

Nationally, the average size of heronries in England is about 18 nests (Marchant, 2000). The heronry at Walthamstow Reservoirs is one of the largest in the country, with 110 nests in 2000 – excluding this site, the average in London is 13. Recently established heronries demonstrate that new sites can be found in London, though the number of suitable locations for future heronries is limited. The only recent instance of a heronry failing to become established was at Kew Gardens (London Borough of Richmond), where two pairs nested between 1989 and 1991.

The Heronries Census shows that severe winters have a major impact on heron numbers (Marchant et al, 1990). Numbers can decline sharply in severe winters and may recover to previous levels within five years. The last severe winter was in 1985/86.

Despite their clear adaptation to busy urban environments, heronries, day roosts and foraging habitat remain vulnerable to disturbance from recreational and development activity. The most vulnerable period is while birds are sitting on eggs and it is often not realised that birds can be incubating as early as February.

Herons that visit garden ponds may not be welcomed by the owners. The extent of any persecution in London is unknown and is most likely to be a problem from individuals keeping expensive Koi carp. Discarded fishing line is also a threat to herons and other wildlife.

The increasing inland cormorant population may become a threat to herons if they compete for the same nesting sites. They began nesting at Walthamstow in 1990 and Broadwater in 1997 and by 2000, there were 240 and 15 nests respectively. At both sites, the herons and cormorants use different islands and at the moment there is no conflict.

Recently, a number of young from a heronry in Nottinghamshire have been badly deformed and died in the nest (Blackburn & Kent, 1999). The cause of these deformities is unknown and the phenomenon has not been observed in London.

The Wildlife and Countryside Act (1981, as amended) protects grey herons from killing and catching, being held in captivity, and the wilful destruction of their nests. It is possible to apply for a licence to kill birds where they are proven to cause serious damage at fisheries.

Each year, volunteers count the nests in London’s heronries for the BTO Heronries Census. The grey heron is included in the BTO/WWT/RSPB/JNCC Wetland Bird Survey (WeBS) and volunteers make monthly counts at many sites in London (see Annex).

The RSPB has produced a free advisory leaflet, ‘Herons and Garden Fish Ponds’ that gives advice on deterring herons from taking fish.