Garceta Dimorfa

Identificación

This bird has two plumage colour forms. There is an all-white morph and a dark grey morph; morphs can also occur with intermediate shades of grey which may be related to age or particoloured in grey and white. The white morph is similar in general appearance to the little egret, but has a larger yellower bill, extended yellow on thicker legs, and when foraging tends to be very active, sometimes also moving its wing or using it to shade the water surface. The grey morph has a whitish throat and is unlikely to be confused with any other species within the range of this egret with beak and legs similar to that of the white morph. During the breeding season the legs and facial skin are reddish. Breeding birds have two long feathers on the sides of the nape. The nominate subspecies gularis has a range from West Africa to Gabon, with some birds breeding in southern Europe. Subspecies schistacea (Hemprich & Ehrenberg, 1828) breeds from the Persian Gulf along the coast of India to the east of the India Peninsula. The bill of gularis is more pointed while schistacea has the larger bill especially towards the base. The form on the eastern coast of South Africa is usually separated as the dimorphic egret Egretta dimorpha. The dark and white morph is thought to be controlled by a single allele with the dark character being incompletely dominant over the gene for white.

Taxonomía

The species was first described as Ardea gularis from a specimen obtained in Senegal by the French naturalist Louis Augustin Guillaume Bosc in 1792. The distinctive white throat (or gular region) in the dark morph gives it the species name. Later authors have treated it the genera Demiegretta (in which dimorphic egrets were once included), Herodias and Lepterodius until its current stable position in the genus Egretta. The distinctness of this species from Egretta garzetta has however been highly debated with some authors treating garzetta as a polytypic superspecies. There has been confusion related to what were thought to be dark little egrets and the possibility of hybrids. Dark morphs in little egrets are extremely rare and nearly all specimens of what were thought to be dark morphs have turned out to be western reef herons. In India some research claimed circumstantial evidence of interbreeding with little egrets, but the same researchers noted that the breeding seasons of the two species were different. Hybridization has also been claimed to occur in Morocco and Kenya (but this might refer to Egretta dimorpha). Christidis and Boles quote a report by McCracken and Sheldon (2002) that the nucleotide sequences of the cytochrome b genes from a sampled little egret and a western reef heron were identical and use this as evidence for demotion. The sequences and the origins of the samples are not publicly available or verifiable. In the past the Indian form which was also described as Ardea asha by William Henry Sykes has been treated as a subspecies of the little egret as Egretta garzetta schistacea on the basis of presumed hybridization with Egretta gularis. The lores of breeding little egrets are blue while those of the reef heron are reddish although some individuals of schistacea show blue and this is thought to be due to hybridization. The dark coastal form of Madagascar, Aldabra, Comoro Islands, Seychelles and parts of East Africa (southern Kenya to Tanzania) was earlier treated as a subspecies (the mainland African form has sometimes been considered as schistacea) but is now raised to full species as the dimorphic egret (Egretta dimorpha).

Nombre científico

Egretta gularis

Autoridad

(Bosc, 1792)

Orden

Pelecaniformes

Familia

Ardeidae

Género

Egretta

Distribución y hábitat

It occurs mainly on the coasts in tropical west Africa, the Red Sea, the Persian Gulf (Iran) extending east to India. It also occurs in the Lakshadweep Islands and Sri Lanka where breeding was once recorded at Chilaw. The nominate subspecies breeds in west Africa from Mauritania to Gabon. Birds may also be found off the mainland such as in the Canary Islands. Small numbers breed in Spain. Subspecies schistacea is found from the Red Sea coast east round the Indian coast. Breeding colonies are known from the east coast of India around Pulicat Lake. They occasionally occur further inland.<p>The western reef heron (nominate subspecies) occurs as a vagrant in North America, South America and the Caribbean islands. Based on the growing number of records it is suspected that they may establish breeding colonies in Brazil. Several records around 1980–90 in Germany, Austria and France have been attributed to birds that escaped from an animal dealer in Mittelfranken.

Migración

Especie residente

Comportamiento

Patrón migratorio: Especie residente.

Conservación

Tendencia poblacional: estable.

Tendencia poblacional

estable