Joe Marshall's Birds of Pine-Oak Woodland in Southern Arizona and Adjacent Mexico is often mentioned as an important early contributor to the terms 'sky island' and 'Madrean archipelago'. From the introduction:
Woodland of mixed pines and oaks is familiar mountain scenery in Mexico, whence it extends into southeastern Arizona along with many kinds of Mexican birds. This woodland occupies a belt from about 5500 to 6500 feet in elevation between encinal (oak woodland) below and ponderosa pine forest above. It combines tree forms of both these zones as to make a smooth transition between them. The present report compares the number of each species of breeding bird in a series of stations, within pine-oak woodland, which were visited in the summers of 1951, 1952, and 1953. These sites extend from the Pinaleno and Santa Catalina mountains in Arizona south into central Sonora and to the Sierra Madre Occidental of northwestern Chihuahua.