John H. Robinson, Our Domestic Birds: Elementary Lessons in Aviculture
“ So in arranging for nests the pigeon keeper builds shelves 10 or 12 inches apart, and divides these into compartments about 12 inches wide, thus forming pigeonholes. Because a hen pigeon often lays again and begins to incubate before a pair of young are ready to leave the nest, it is usual to arrange the pigeonholes in pairs. ”
