Wandering through a Florida meadow in the spring of 1952, amateur Birdwatcher Richard Borden spotted a curious sight: among a grazing herd of cattle was a flock of yellow-legged, short-necked white herons, darting between the cows' legs, snaring grasshoppers flushed up from the pasture. Borden casually shot a series of pictures, mistaking the birds for snowy egrets, a common Florida species. Months later, Borden discovered he had the first pictures ever taken of a new U.S. immigrant: the Old World's buff-backed, yellow-billed cattle egret (Bubulcus ibis).
The egrets' migration to the Western Hemisphere is one of today's most fascinating ornithological puzzles. Never...