African Egrets? Holy Cow!
Line, Les, International Wildlife
CATTLE EGRETS--big surprise!--like to hang out with cows. In the soggy pastures of Florida, first stop on this Old World heron's amazing invasion of North America, the birds run with lean and mean Brahmans and stroll with fat and contented Herefords. In crowded India, where there are more cows per capita than in any country in the world, cattle egrets nest in spreading mango trees in the middle of chaotic bazaars so they can be close to village herds.
In the eyes of a cattle egret, a cow is a cow is a cow, and the breed matters naught. Cattle are a free meal ticket because their shuffling feet stir up the insects that these gregarious birds devour at a rate of two or three a minute. Scientists call this commensal feeding, which means that the egrets profit from the relationship but the cows don't. Think of cattle as beaters pushing through the grass, driving out game for the white hunters.
On every count--availability, suitability and reliability--cows are the perfect hosts for these birds, says ornithologist Joanna Burger of Rutgers University in New Jersey. And cows apparently were the egret's passport to a phenomenal global range expansion earlier in this century. The pronounced tendency of young cattle egrets to scatter to the four winds after the breeding season--one banded bird traveled 4,200 kilometers (2,600 mi.) from its natal rookery--also played a critical role. But as Burger points out, the species' population explosion coincided with a worldwide boom in intensive cattle farming and in the creation of pasturelike habitats in regions once uninviting to this terrestrial heron.
Unlike other herons, cattle egrets are not shackled to aquatic habitats. Though they nest in places like mangrove swamps and regularly forage along the grassy margins of lakes, marshes and rivers for such semiaquatic prey as frogs, the key to their survival is an abundance of insect life.
For example, in tropical Africa, where the cattle egret is believed to have evolved in partnership with wild grazing animals, the birds congregate on floodplains that slowly dry after being inundated by seasonal rains. These wet grasslands attract a wide variety of insects that provide the herons with a year-round source of food. Africa's plains and savannahs, on the other hand, are largely inhospitable to egrets except during locust outbreaks, when thousands of the birds descend on the grasshopper swarms.
Some of Africa's drier regions did, however, become habitat for the nomadic cattle egret starting in this century, when farmers converted more arid landscapes to irrigated grain fields and livestock production. The birds then spread across southern Europe and conquered the Americas. Meanwhile, with a similar boost from agricultural development, the Asian race of the cattle egret (a huge gap in the species' distribution occurs in parts of the Middle East) expanded its breeding range into China and Japan, south to Australia (where the birds consort with kangaroos) and on to New Zealand. A few adventuresome cattle egrets even reached Alaska, Iceland and Antarctica, although they haven't nested in such cold climes--at least, not yet.
Seize the Opportunity
Burger, who has studied cattle egrets from New Jersey to South Africa, notes that the species shares a history of continental conquest with the Norway rat and house mouse, which also benefited from human agricultural activities. No one, however, would otherwise compare this ivory-white heron to a grungy, naked-tailed rodent.
Hunched next to a lithe snowy egret, a native of New World marshes, the chicken-size cattle egret appears rather short and stocky. When foraging alongside a cow, the bird sways its head and neck back and forth like a snake mesmerizing a frog. In the breeding season, however, the cattle egret is transformed into a particularly handsome bird, flaunting orangish plumes on its crown, mantle and lower neck; its bill, legs and irises turn flame red for a period of two or three weeks before egg-laying begins. …