The adult eastern cattle egret has few predators, but birds or mammals may raid its nests, and chicks may be lost to starvation, calcium deficiency or disturbance from other large birds. This species maintains a special relationship with cattle, which extends to other large grazing mammals; wider human farming is believed to be a major cause of their suddenly expanded range. The cattle egret removes ticks and flies from cattle and consumes them. This benefits both species, but it has been implicated in the spread of tick-borne animal diseases.
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.
The eastern cattle egret differs from the western species in breeding plumage, when the buff colour on its head extends to the cheeks and throat, and the plumes are more golden in colour. The bill and tarsus are longer on average. It is also smaller and shorter-winged. It has white cheeks and throat, but the nuptial plumes are golden. 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.
This species gives a quiet, throaty rick-rack call at the breeding colony, but is otherwise largely silent.
In Australia, colonisation began in the 1940s, with the species establishing itself in the north and east of the continent. It began to regularly visit New Zealand in the 1960s.
The expansion of the eastern 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.
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.
No comments:
Post a Comment