The cattle egret was originally a bird of Africa and southern Asia, following large grazing mammals and catching the grasshoppers and flies they flush. Its first recorded appearance in the New World was in the Guianas in the late nineteenth century, most plausibly by birds carried across the narrow part of the Atlantic on winds.
What followed was rapid. The species reached Florida in the 1940s, bred there in the 1950s, and spread through North America to southern Canada within a few decades, while also expanding through South America and, separately, into Australia and New Zealand. The engine of the spread was cattle ranching: clearing forest for pasture manufactured exactly the habitat the bird needed, and it simply moved into it.
The spread is often described as an invasion, which is imprecise. No human transport is implicated, the birds arrived by natural dispersal, and evidence of harm to native herons is limited. Whether it competes meaningfully with native species remains an open question.