Hidden high in the Sierra de Guatemala mountain range of northeastern Mexico in the state of Tamaulipas is the northernmost tropical cloud forest of the Western Hemisphere. Within its humid oak-sweetgum woodlands, tropical and temperate species of plants and animals mingle in rare diversity, creating a mecca for birders and other naturalists.

Fred and Marie Webster first visited Rancho del Cielo, cloud forest home of Canadian immigrant Frank Harrison, in 1964, drawn by the opportunity to see such exotic birds as tinamous, trogons, motmots, and woodcreepers only 500 miles from their...