Heron Taxonomy and Evolution
The herons are a fairly ancient group of birds. Although bird fossils are rare, herons are exceptionally rare even by avian standards totaling fewer than 40 identified species. Herons first emerge in the fossil record some 60-38 million years ago. Birds attributable to contemporary genera first occur about 7 million years ago. These include Nycticorax, Ardea, Egretta and Ardeola. Thus by the Miocene, a period characterized by extensive aquatic habitats, herons rather closely resembling modern forms had evolved and had radiated into the kinds of herons known today, day and night herons, and large, medium and small herons. Fossils are commoner in more recent times. Those from the Pleistocene are often assignable to extant species, so contemporary heron species have been around a long time. Subfossils from islands often represent recently extinct forms, species or subspecies, particularly of Nycticorax. Most of these went extinct when humans colonized their islands. These fossils are illustrative of the ever-changing face of heron distribution in the world. Herons can make significant changes in range within a matter of decades. As a result, conservation depends on an accurate monitoring of herons’ ranges and with special attention to isolated and outlying populations.
Most extant species are part of a single subfamily of “typical herons,” the Ardeinae. This subfamily includes the herons and egrets (Ardea, Egretta), the green herons (Butorides), the pond herons (Ardeola), a few monotypic genera, but also the night herons (Nycticorax, Gorsachius). Within the typical heron subfamily, three groups are recognized as tribes, Ardeini, Egrettini, and Nycticoraxini. The second subfamily, Botaurinae, encompasses the bitterns (Ixobrychus, Botaurus, Zebrilus). These birds share a number of morphological and behavioural characteristics that differ from the typical herons. Several species are quite distinctive from other herons, and from each other. These are the Boat Billed Heron, the tiger herons, and the Agami Heron, all of which are allocated to separate Subfamilies. It is thought they represent ancient lineages of herons, and so are of special conservation concern.
At the genus level, heron taxonomy is in a state of flux, and has been for decades. WCA recognizes 17 genera. Several species represent monotypic genera, and so are of conservation special interest, Pilherodius, Syrigma, Agamia, Zebrilus, Zonerodius, Tigriornis, Nyctinassa, and Cochlearius.
At the species level, many herons are well characterized. But there remains much uncertainty as to the species limits of many forms. The green herons, the Little Egret group, Intermediate Egret, great egrets, pond herons, small bitterns, large bitterns, Boat Billed Heron, Great Blue Heron and Grey Heron are some of the forms for which species limits remain unsettled. As late as 2005, a new heron species was recognized as being distinct from another. Of course this uncertainty may reflect in part difficulty with applying the species concept to widespread forms with isolated populations. But it more fundamentally reflects a lack of behavioural and molecular information available on these birds. Research is essential to properly design conservation actions.
Ardea includes the largest modern heron, Ardea goliath, and other ‘giant’ herons, all of which are of conservation concern. The White-bellied Heron is among the world’s most rare and endangered. The three large Ardea (cinerea, herodias and cocoi) appear closely related. The nearly cosmopolitan Cattle Egret (previously placed in Bubulcus, Ardeola, or Egretta), the Great Egret and the Eastern Great White Egret (previously referred to Egretta and Casmerodius as well as Ardea), and the Intermediate Egret (previously Egretta), also are Ardea. Shifting names may cause some confusion among conservationists. There remain a number of interesting research questions and uncertainties within Ardea. These include the possible specific distinctiveness of certain populations, including Ardea cinerea monicae in Mauritania and Ardea herodias in southern Florida and the Caribbean and the north Pacific coast of North America. It remains unclear how many species are in the Great Egret group. Although the Great Egret and Eastern Great Egret are currently recognized, it is unclear what is the correct delineation of species in this nearly cosmopolitan group. Conservation action centering on these populations is critical.
Closely related to Ardea are the Butorides and Ardeola herons. As a whole the Butorides herons demonstrate a degree of geographic and individual variation in aspects of plumage unparalleled in extant herons, with many subspecies being recognized. Butorides is vagile, with a new population being established in Bermuda within the last decade. The species limits within this genus have been a huge problem for decades, particularly as to whether the Green Heron and Striated Heron are or are not the same species. This is a genus for which conservation of geographic populations is especially important, irrespective of what the current taxonomic opinions might be.
The birds commonly called egrets from the courtship feathers many possess, the Egretta, pose a taxonomic difficulty in that so many are have white plumage, and so may actually be unrecognized separate species. There are many taxonomic problems within Egretta that require study in order to designate populations of conservation interest. No one is more confusing than that of the various Little Egrets, a population of which recently became established in the Western Hemisphere. One species is currently recognized, the Little Egret (Egretta garzetta), but some of its populations have more frequently been recognized as being distinctive, such as the Egretta dimorpha and Egretta gularis. The final resolution of speciation within this group remains to be completed. Other species also within the typical heron subfamily are three New World species, Syrigma, Pilherodius, and Nyctinassa. Gorsachius includes three species of solitary nesting Old World herons but are little understood and include one of the most endangered of herons. Nycticorax is a widely dispersed group of two extant colonial species, the Black Crowned and Rufous Night Herons. Night herons have in the past established populations on isolated islands. There once were additional species of Nycticorax, which have gone extinct on islands in the Pacific, Indian and Atlantic oceans in past 5,000 to 300 years. Conservation of island populations is a high priority, irrespective of current taxonomy.
Within the second heron subfamily, the bitterns, Botaurus includes four superficially similar species in, North America, South America, Australia and Eurasia-Africa. All species are of conservation concern. Eight small bitterns are included in the genus Ixobrychus. The most distinctive may be the little known Zigzag Heron, which is of conservation concern due to the lack of distribution and status information.
One of the interesting and critical recent advances in understanding the systematics of herons is that three groups of herons are more primitive than the rest, the tiger herons, the Agami Heron, and the Boat Billed Heron, the latter being a sister group of all the remaining herons. The tiger heron subfamily includes five species. Members of the group have somewhat bittern-like plumage, but differ from bitterns in a number of skeletal characteristics and in behavior. The distributions of the species are scattered in the tropics of America (Tigrisoma), New Guinea (Zonerodius) and Africa (Tigrionis). The Old World forms are of conservation concern. The Agami Heron is a unique and little known heron from the American tropics. The Boat Billed Heron is a polytypic species, also unique and requiring additional study to understand its population structure for conservation purposes.