Western Cattle-Egret

识别特征

The cattle egret is a stocky heron with an 88–96 cm (35–38 in) wingspan; it is 46–56 cm (18–22 in) long and weighs 270–512 g (9.5–18.1 oz). It has a relatively short thick neck, a sturdy bill, and a hunched posture. The non-breeding adult has mainly white plumage, a yellow bill and greyish-yellow legs. During the breeding season, adults of the nominate western subspecies develop orange-buff plumes on the back, breast and crown, and the bill, legs and irises become bright red for a brief period prior to pairing. The sexes are similar, but the male is marginally larger and has slightly longer breeding plumes than the female; juvenile birds lack coloured plumes and have a black bill.<p>The positioning of the egret's eyes allows for binocular vision during feeding, and physiological studies suggest that the species may be capable of crepuscular or nocturnal activity. Adapted to foraging on land, they have lost the ability possessed by their wetland relatives to accurately correct for light refraction by water.<p>This species gives a quiet, throaty rick-rack call at the breeding colony, but is otherwise largely silent.

分类信息

The cattle egret was first described in 1758 by Linnaeus in his Systema naturae as Ardea ibis, but was moved to its current genus by Charles Lucien Bonaparte in 1855. Its genus name Bubulcus is Latin for herdsman, referring, like the English name, to this species' association with cattle. Ibis is a Latin and Greek word which originally referred to another white wading bird, the sacred ibis, but was applied to this species in error.<p>Despite superficial similarities in appearance, the cattle egret is more closely related to the genus Ardea, which comprises the great or typical herons and the great egret (A. alba), than to the majority of species termed egrets in the genus Egretta. Rare cases of hybridization with little blue herons Egretta caerulea, little egrets Egretta garzetta and snowy egrets Egretta thula have been recorded.

学名

Ardea ibis

命名信息

(Linnaeus, 1758)

目

Pelecaniformes

科

Ardeidae

属

Ardea

分布与栖息地

The cattle egret has undergone one of the most rapid and wide reaching natural expansions of any bird species. It was originally native to parts of Southern Spain and Portugal, tropical and subtropical Africa and humid tropical and western Asia. In the end of the 19th century it began expanding its range into southern Africa, first breeding in the Cape Province in 1908. Cattle egrets were first sighted in the Americas on the boundary of Guiana and Suriname in 1877, having apparently flown across the Atlantic Ocean. It was not until the 1930s that the species is thought to have become established in that area.<p>The species first arrived in North America in 1941 (these early sightings were originally dismissed as escapees), bred in Florida in 1953, and spread rapidly, breeding for the first time in Canada in 1962. It is now commonly seen as far west as California. It was first recorded breeding in Cuba in 1957, in Costa Rica in 1958, and in Mexico in 1963, although it was probably established before that. In Europe, the species had historically declined in Spain and Portugal, but in the latter part of the 20th century it expanded back through the Iberian Peninsula, and then began to colonise other parts of Europe; southern France in 1958, northern France in 1981 and Italy in 1985. Breeding in the United Kingdom was recorded for the first time in 2008 only a year after an influx seen in the previous year. In 2008, cattle egrets were also reported as having moved into Ireland for the first time.<p>The massive and rapid expansion of the cattle egret's range is due to its relationship with humans and their domesticated animals. Originally adapted to a commensal relationship with large grazing and browsing animals, it was easily able to switch to domesticated cattle and horses. As the keeping of livestock spread throughout the world, the cattle egret was able to occupy otherwise empty niches. Many populations of cattle egrets are highly migratory and dispersive, and this has helped the species' range expansion. The species has been seen as a vagrant in various sub-Antarctic islands, including South Georgia, Marion Island, the South Sandwich Islands and the South Orkney Islands. A small flock of eight birds was also seen in Fiji in 2008.<p>In addition to the natural expansion of its range, cattle egrets have been deliberately introduced into a few areas. The species was introduced to Hawaii in 1959. Successful releases were also made in the Seychelles and Rodrigues, but attempts to introduce the species to Mauritius failed. Numerous birds were also released by Whipsnade Zoo in England, but the species was never established.<p>Although the cattle egret sometimes feeds in shallow water, unlike most herons it is typically found in fields and dry grassy habitats, reflecting its greater dietary reliance on terrestrial insects rather than aquatic prey.

迁徙

Complete migrants

保护状况

种群趋势

rising