Belongs within: Ardeidae.
The genus Ardea contains the larger herons. Ardea species are generally (but not always) darker than the egrets of the genus Egretta, and lack powder down patches on the thighs. Most are solitary nesters.
Authors have differed over the assignment of some heron species to Ardea or related genera but the core members of Ardea include species related to the grey heron A. cinerea and the purple heron A. purpurea. The grey heron is widely distributed in Eurasia and Africa; it is mostly white on the head, neck and underparts, and grey on the back, with a black superciliary stripe on the head leading to a thin black crest. The similarly distributed (though not spreading as far north) purple heron has a darker grey back and chestnut hindneck. The great blue heron A. herodias of North America is similar in appearance to the grey heron but generally larger. The cocoi heron A. cocoi of South America has a grey back and wings, white neck and underparts, and a black crown and nape.
<==Ardea Linnaeus 1758 [=Ardaea Dumont in Levrault 1804; incl. Myola Mathews 1913]CC10
|--A. pacifica Latham 1802JT12, CC10 (see below for synonymy)
|--A. purpurea Linnaeus 1766BKB15, CS77
| |--A. p. purpureaCS77
| |--A. p. bournei de Naurois 1966CS77
| |--A. p. madagascariensis van Oort 1910CS77
| `--A. p. manilensis Meyen 1834CS77
`--+--A. herodias Linnaeus 1758BKB15, CS77
| |--A. h. herodiasS18
| `--A. h. lessoniiS18
`--+--*A. cinerea Linnaeus 1758CC10, BKB15, CC10
| |--A. c. cinereaCC10
| |--A. c. firasa Hartert 1917CS77
| |--A. c. jouyi Clark 1907CC10 [incl. A. altirostris Mees 1971CS77]
| `--A. c. monicae Jouanin & Roux 1963CS77
`--A. cocoiBKB15
Ardea incertae sedis:
A. aequinoctialis Linnaeus 1758L58
A. agami [incl. Agamia picta]SS66
A. antigone Linnaeus 1758L58
A. atricapillaS66
A. atricollisS66
A. aurelianensis Milne-Edwards 1871M02
A. caerulea Linnaeus 1758L58
A. goliath Cretzschmar 1826CS77
A. humblotiJT12
A. imperialisSU93
A. insignisJT12
A. leucoptera [incl. A. idae]S66
A. maculata Gmelin 1789 [=Ardetta maculata]CC10
A. majorF43
A. melanocephala Vigors & Children 1826CS77
A. modesta Gray 1831 (see below for synonymy)CC10
A. nivea Gmelin 1770CC10
A. occidentalisRN72
A. pavonina Linnaeus 1758L58
A. picata Gould 1845JT12, WS48 [=A. (Herodias) picataWS48, Notophoyx picataWS48]
A. piveteaui Brunet 1970M02
A. rectirostris Gould 1843 [=A. cinerea rectirostris]CC10
A. rupeliensis Beneden 1873M02
A. schistaceaS66
A. sturmi [=Ardetta sturmi]K08
A. sumatrana Raffles 1822WS48
|--A. s. sumatranaWS48
`--A. s. mathewsae Mathews 1912 [incl. A. s. gilberti Mathews 1912]WS48
A. thulaF43
A. violacea Linnaeus 1758L58
A. virescens Linnaeus 1758L58
A. virgo Linnaeus 1758L58
Nomina nuda: Ardea effosa Meyer in Lepsius 1883M02
A. latipes Meyer in Lepsius 1883M02
Ardea modesta Gray 1831 [=A. alba modesta, Egretta alba modesta; incl. A. alba Ellman 1861 non Linnaeus 1758, Herodias alba maoriana Mathews & Iredale 1913, Casmerodius albus maorianus, Egretta alba neglecta Mathews 1912, Ardea putea Franklin 1831, Herodias syrmatophorus Gould 1846, A. syrmatophora, Egretta alba syrmatophora, Herodias alba syrmatophora, Ardea timoriensis Lesson 1831, H. timoriensis, A. torra Franklin 1831]CC10
Ardea pacifica Latham 1802JT12, CC10 [=*Myola pacificaCC10, Notophoyx pacificaCC10, Ardea bullaragang Wagler 1827CC10, A. bullaranjausCC10]
*Type species of generic name indicated
References
[BKB15] Burleigh, J. G., R. T. Kimball & E. L. Braun. 2015. Building the avian tree of life using a large-scale, sparse supermatrix. Molecular Phylogenetics and Evolution 84: 53–63.
[CC10] Checklist Committee (OSNZ). 2010. Checklist of the Birds of New Zealand, Norfolk and Macquarie Islands, and the Ross Dependency, Antarctica 4th ed. Ornithological Society of New Zealand and Te Papa Press: Wellington.
[CS77] Cramp, S., & K. E. L. Simmons (eds) 1977. Handbook of the Birds of Europe, the Middle East and North Africa: The Birds of the Western Palaearctic vol. 1. Ostrich to Ducks. Oxford University Press: Oxford.
[F43] Fraser, L. 1843. On the collection of birds brought to England by Mr. Bridges. Proceedings of the Zoological Society of London 11: 108–121.
[JT12] Jetz, W., G. H. Thomas, J. B. Joy, K. Hartmann & A. Ø. Mooers. 2012. The global diversity of birds in space and time. Nature 491: 444–448.
[K08] Kellogg, V. L. 1908. Wissenschaftliche Ergebnisse der Schwedischen Zoologischen Expedition nach dem Kilimandjaro, dem Meru und den Umgebenden Massaisteppen Deutsch-Ostafrikas 1905–1906 vol. 15. Corrodentia pt 4. Mallophaga. Almqvist & Wiksells Boktryckeri-A. B.: Uppsala.
[L58] Linnaeus, C. 1758. Systema Naturae per Regna Tria Naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Laurentii Salvii: Holmiae.
[M02] Mlíkovský, J. 2002. Cenozoic Birds of the World. Part 1: Europe. Ninox Press: Praha.
[RN72] Rutgers, A., & K. A. Norris (eds) 1972. Encyclopaedia of Aviculture vol. 1. London, Blandford Press.
[S66] Schlegel, H. 1866. Communication from, on mammals and birds collected in Madagascar. Proceedings of the Zoological Society of London 1866: 419–426.
[SS66] Sclater, P. L., & O. Salvin. 1866. On some additions to the catalogue of birds collected by Mr. E. Bartlett on the River Ucayali. Proceedings of the Zoological Society of London 1866: 566–567.
[SU93] Sonobe, K., & S. Usui (eds.) 1993. A Field Guide to the Waterbirds of Asia. Wild Bird Society of Japan: Tokyo.
[S18] Stone, W. 1918. Birds of the Panama Canal Zone, with special reference to a collection made by Mr. Lindsey L. Jewel. Proceedings of the Academy of Natural Sciences of Philadelphia 70: 239–280.
[WS48] Whittell, H. M., & D. L. Serventy. 1948. A systematic list of the birds of Western Australia. Public Library, Museum and Art Gallery of Western Australia, Special Publication 1: 1–126.