Western cattle egret

Appearance

The cattle egret is a stocky heron with an 88–96 cm wingspan; it is 46–56 cm long and weighs 270–512 g. 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.

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.

Distribution

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.

Status

This species has a large range, with an estimated global extent of occurrence of 10,000,000 km2. On the other hand, the expansion and establishment of the species over large ranges has led it to be classed as an invasive species.

Behavior

This species gives a quiet, throaty ''rick-rack'' call at the breeding colony, but is otherwise largely silent.

Habitat

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.

Reproduction

The cattle egret nests in colonies, which are often, but not always, found around bodies of water. The colonies are usually found in woodlands near lakes or rivers, in swamps, or on small inland or coastal islands, and are sometimes shared with other wetland birds, such as herons, egrets, ibises and cormorants. The North American breeding season lasts from April to October. In the Seychelles, the breeding season of the subspecies ''B.i. seychellarum'' is April to October.

The male displays in a tree in the colony, using a range of ritualised behaviours such as shaking a twig and sky-pointing , and the pair forms over three or four days. A new mate is chosen in each season and when re-nesting following nest failure. The nest is a small untidy platform of sticks in a tree or shrub constructed by both parents. Sticks are collected by the male and arranged by the female, and stick-stealing is rife. The clutch size can be anywhere from one to five eggs, although three or four is most common. The pale bluish-white eggs are oval-shaped and measure 45 mm × 53 mm . Incubation lasts around 23 days, with both sexes sharing incubation duties. The chicks are partly covered with down at hatching, but are not capable of fending for themselves; they become capable of regulating their temperature at 9–12 days and are fully feathered in 13–21 days. They begin to leave the nest and climb around at 2 weeks, fledge at 30 days and become independent at around the 45th day.

The cattle egret engages in low levels of brood parasitism, and there are a few instances of cattle egret eggs being laid in the nests of snowy egrets and little blue herons, although these eggs seldom hatch. There is also evidence of low levels of intraspecific brood parasitism, with females laying eggs in the nests of other cattle egrets. As much as 30% extra-pair copulations have been noted.

The dominant factor in nesting mortality is starvation. Sibling rivalry can be intense, and in South Africa third and fourth chicks inevitably starve. In the dryer habitats with fewer amphibians the diet may lack sufficient vertebrate content and may cause bone abnormalities in growing chicks due to calcium deficiency. In Barbados, nests were sometimes raided by vervet monkeys, and a study in Florida reported the fish crow and black rat as other possible nest raiders. The same study attributed some nestling mortality to brown pelicans nesting in the vicinity, which accidentally, but frequently, dislodged nests or caused nestlings to fall.

Food

The cattle egret feeds on a wide range of prey, particularly insects, especially grasshoppers, crickets, flies, and moths, as well as spiders, frogs, and earthworms.