Great blue heron  (Ardea herodias)

Classified as Least Concern (LC) on the IUCN Red List (1).

At over a metre and half in height, the great blue heron is the largest heron in North America and one of the continent’s most recognisable wading birds (2) (3). There are two main colour morphs of this statuesque species: a dark form that is mostly blue-grey, with chestnut thighs, and a white cap over a black eye stripe that merges into long, black plumes; and a light form which is all white. The neck, legs and wings are characteristically long, the tail is short, and the yellowish bill is thick, elongate and tapered. In flight, it coils its neck back into a distinctive s-shape, extends its legs back along its body axis, and beats its wings with steady, powerful strokes (2) (3) (4). Although the sexes are similar in appearance, the female is normally around ten percent smaller than the male, while juveniles are duller and lack the long plumes of the adults (4) (5). Considerable uncertainty surrounds the separation of subspecies of the great blue heron, with between two and seven recognised for North America alone (4). However, five main subspecies, that differ in size and plumage and occupy different parts of the species overall range, are commonly referred to: Ardea herodias herodias, A. h. fannini, A. h. wardii, A. h. occidentalis (the white form), and A. h. cognata (2) (4).

The great blue heron breeds throughout much of North America, Central America, and on numerous Caribbean Islands, and the Galapagos (4). The subspecies Ardea herodias herodias occurs over most of North America; A. h. fannini occupies the northwest of North America; A. h. wardii is found in the eastern USA, from Kansas to Florida; A. h. occidentalis inhabits extreme southern Florida, the Caribbean and Mexico; and A. h. cognata is restricted to the Galapagos (2) (4).

Found in a diverse range of habitats including salt marsh and mangrove swamps, freshwater marshes and swamps, estuaries, coastal lagoons, flooded fields, ditches, riverbanks, and lake edges (4).

Foraging alone or in flocks, the great blue heron usually hunts by slowly wading or standing motionless in shallow water (2) (4). Although fish form the bulk of its diet, it will also take amphibians, invertebrates, reptiles, mammals and birds. When prey is sighted, it rapidly thrusts its neck forward to snatch the hapless quarry within its bill, and then typically swallows it whole (2) (3). As an alternative to walking or standing, it also occasionally exhibits a range of other foraging techniques that enable it to access deeper water where it is unable to wade. This includes hovering above the water, plunging beneath it, and simply swimming on the surface (4).

Generally, the great blue heron nests in tall trees that are near to aquatic feeding areas, and are to some extent isolated from human disturbance (4). However, when trees are not available, it will also nest on the ground in areas free from predators, and in reeds, shrubs and mangroves (2). Although some nest singly, many breed in colonies, which vary in size depending on the amount of nearby foraging habitat (2) (4). Mates and nest sites change from year to year, with the consequence that mate selection is a critical part of the yearly cycle (4). Usually the male secures a display site first, such as an old nest, before engaging in an elaborate courtship display that includes an impressive repertoire of stretching, snapping, twig shaking, crest raising, and circling flight. After forming a pair and mating, the female lays between two to seven pale blue eggs, which are then incubated by both parent birds for around 28 days. The young are fed regurgitated food and fledge the nest after around 60 days (2) (4).

The seasonal movements of the great blue heron is very much dependant on its location (3) (4). Herons from north-central North America migrate south over winter, whilst those closer to the either coast are more variable in their movements, with some remaining year round in the same location (2) (4).

The great blue heron has a remarkable ability to exploit a wide range of habitats and food types. Consequently it is widespread and abundant, and is not subject to any major threats at the species level (4) (6). Nonetheless, some populations, particularly those occupying small areas on the coast, are vulnerable to localised impacts (4). This broadly includes habitat destruction, human disturbance and persecution, and contamination by pollutants (2) (4).

Critical to the conservation of threatened populations of the great blue heron is the protection of nesting sites and feeding habitats, and the conservation of food supplies (2) (4). This is already being achieved by initiatives such as the Heron Working Group, the primary goal of which is to ensure a viable and self-sustaining population of the Pacific great blue heron (A. h. fannini) (7).

View information on this species at the UNEP World Conservation Monitoring Centre.

To find out more about the great blue heron see:

• National Audubon Society:
  www.audubon.org
• The Birds of North America Online:
  http://bna.birds.cornell.edu

To find out more about the conservation of the Pacific great blue heron see:

• The Heron Working Group:
  www.heronworkinggroup.org

For more information on this and other bird species please see:

• BirdLife International:
  http://www.birdlife.org