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.