The cattle egret (Bubulcus ibis) is a member of the Ardeidae (heron family) included in the order Ciconiiformes (stork-like birds). It is gregarious and usually associates with cattle and other grazing animals. Adults are about seventeen inches (43.2 cm) in length, have a wingspan of about thirty-seven inches (94.0 cm), and weigh between 0.6 and 1.0 pound (0.3–0.5 kg). Their plumage is white, but during the breeding season orange-buff plumes appear on the head, neck, and back. Cattle egrets are native to Africa and Asia. The nature and success of their almost worldwide range expansion is dynamic and complex, but well-documented. Although the mechanisms and details of original range expansion are not known, cattle egrets spread from the west coast of Africa in the late 1800s across the Atlantic Ocean to the coastal area of northeastern South America, then northward into North America and the West Indies in the early 1940s and 1950s. Cattle egrets originally entered Texas in 1954, migrating south and west along the Gulf Coast states from initial heronries in Florida. They increased from about ten pairs in the state in 1959 to about 300,000 pairs by 1990. By 1995 they occupied 266 heronries in Texas, mostly along the coast and east of the Balcones Escarpment, but between 1987 and 1995, a few small scattered colonies nested in the South Plains, Trans-Pecos, and Panhandle.
Cattle egrets nest in woodlands and swamps and on inland and coastal islands. There is no correlation between distribution or density of grazing cattle and the breeding range of cattle egrets. In inland heronries, they associate primarily with the little blue heron (Egretta caerulea) and snowy egret (E. thula). In coastal heronries, they primarily associate with the snowy egret and tricolored heron (E. tricolor). They are generally beneficial because of their insect-eating habits. However, in nesting colonies, the deposition of cattle egret guano changes soil chemistry. Some plant species, such as hackberry, cedar elm, and chinaberry, can survive the changes, but others, such as oak, pecan, and winged elm, are killed. Cattle egrets nest about three weeks later than native herons and egrets; their breeding season is seven to nine weeks longer, and they are less selective of nest sites. Their nests consist of twigs and are bowl-shaped. The average clutch size is 3.4; egg-laying intervals are about two days, and the incubation period is about twenty-four days. Cattle egrets lose about 14 percent of their eggs, but hatching failure of remaining eggs is low (7 percent). Chick mortality is about 4 percent. Thus, 2.5 young are fledged per brood.
Mixed clutches of cattle egrets and native ardeids are occasionally found. But since cattle egrets nest later than native herons and egrets, they are mostly noncompetitive with the natives, and interspecific aggression is relatively low. Cattle egrets reuse abandoned nests or take them apart for materials for their own nests, a behavior which is also common among native herons and egrets. From 1959 to 1972 the cattle egret population increased at an average rate of 180 percent a year. Between 1972 and 1990 the rate of increase dropped to an average of 120 percent per year. This decrease may indicate that the bird is reaching its maximum population in Texas.
Annual mortality rates among age groups are: juveniles (56 percent), yearlings (30 percent), and adults (28 percent). Few birds (about 4 percent) live longer than 7–8 years. Maximum life span is unknown, but may reach 20 years. Mortality factors are natural causes such as predation, diseases, injuries, and accidents and human causes such as shooting, trapping, being struck by farm machinery, and entanglement in fishing line. The major cause of mortality is shooting, most of which occurs when migrating birds disperse into Mexico during the winter. Agricultural pesticide residues have been found in eggs and bird tissues in Texas and Mexico; however, there is no indication of adverse effects to cattle egret populations.
Most cattle egrets hatched in Texas winter in Mexico-in the Gulf states of Tamaulipas and Vera Cruz and in the Pacific states of Sinaloa, Michoacán, Jalisco, and Najarit. Band recoveries and sight records of colored leg tags indicate that few cattle egret adults return to natal Texas colonies, but most disperse to other heronries, some at considerable distances from the natal heronry (in Mexico and Central America, for instance). Similar records indicate that they move as far as extreme southern and southwestern Mexico, Guatemala, El Salvador, and Cocos Island (Costa Rica). A few have also been noted in Arkansas, California, Florida, and Louisiana. Compared to the large breeding population in Texas, the winter population of cattle egrets in the state is extremely small with most of them in coastline counties, increasing southward. Sightings away from coastal areas are inconsistent and numbers are small; occasionally, single birds are seen far inland during mild winters.
Cattle egrets obtain 95 percent of their food in association with grazing animals. Major prey items are grasshoppers and cattle-associated flies. Rare prey items include ticks, earthworms, crayfish, fish, and small birds. Frogs and toads are apparently important food items for chicks. After the breeding season, cattle egrets often feed in cotton and grain fields and follow farm equipment. Lack of food may limit their winter distribution in Texas. Consumption of grasshoppers and cattle-associated flies may be of great economic benefit to cattlemen. Cattle egrets reduce the number of tabanids and, thereby, the incidence of bovine anaplasmosis. Laboratory tests have shown that cattle egrets are not carriers of brucellosis, as was once suspected. Heronries near human habitation cause noise, odor, and fear of diseases such as ornithosis and histoplasmosis. There is no evidence, however, that cattle egrets have introduced diseases or parasites detrimental to humans or native herons and egrets.