The herons are the long-legged freshwater and coastal birds in the family Ardeidae, with 64 recognised species, some of which are referred to as "egrets" or "bitterns" rather than herons. Members of the genera Botaurus and Ixobrychus are referred to as "bitterns", and, together with the zigzag heron or zigzag bittern in the monotypic genus Zebrilus, form a monophyletic group within the Ardeidae. Egrets are not a biologically distinct group from the herons, and tend to be named differently because they are mainly white or have decorative plumes. Although egrets have the same build as herons, they tend to be smaller. Herons, by evolutionary adaptation, have long beaks.
The classification of the individual heron/egret species is fraught with difficulty, and there is still no clear consensus about the correct placement of many species into either of the two major genera, Ardea and Egretta. Similarly, the relationship of the genera in the family is not completely resolved. However, one species formerly considered to constitute a separate monotypic family Cochlearidae, the boat-billed heron, is now regarded as a member of the Ardeidae.
Although herons resemble birds in some other families, such as the storks, ibises, spoonbills and cranes, they differ from these in flying with their necks retracted, not outstretched. They are also one of the bird groups that have powder down. Some members of this group nest colonially in trees, while others, notably the bitterns, use reed beds.
The herons are medium to large sized birds with long legs and necks. They exhibit very little sexual dimorphism in size. The smallest species is usually considered the little bittern, which can measure under 30 cm (12 in) in length, although all the species in the Ixobrychus genus are small and many broadly overlap in size. The largest species of heron is the Goliath heron, which stand up to 152 cm (60 in) tall. The necks are able to kink in an S-shape, due to the modified shape of the sixth vertebrae. The neck is able to retract and extend, and is retracted during flight, unlike most other long-necked birds. The neck is longer in the day herons than the night herons and bitterns. The legs are long and strong and in almost every species are unfeathered from the lower part of the tibia (the exception is the zigzag heron). In flight the legs and feet are held backward. The feet of herons have long thin toes, with three forward pointing ones and one going backward.
The bill is generally long and harpoon like. It can vary from extremely fine, as in the agami heron, to thick as in the grey heron. The most atypical bill is owned by the boat-billed heron, which has a broad thick bill. The bill, as well as other bare parts of the body, is usually yellow, black or brown coloured, although this colour can vary during the breeding season. The wings are broad and long, exhibiting 10–11 primaries feathers (the boat-billed heron has only nine), 15–20 secondaries and 12 rectrices (10 in the bitterns). The feathers of the herons are soft and the plumage is usually blue, black, brown, grey or white, and can often be strikingly complex. Amongst the day herons there is little sexual dimorphism in plumage (except in the pond-herons); differences between the sexes are the rule for the night herons and smaller bitterns. Many species also have different colour morphs. In the Pacific reef heron there are both dark and light colour morphs, and the percentage of each morph varies geographically. White morphs only occur in areas with coral beaches.