Structure for keeping captive birds, usually spacious enough for the aviculturist to enter. Aviaries range from small enclosures to large flight cages 100 ft (30 m) or more long and up to 50 ft (15 m) high. Enclosures for birds that fly only little or weakly (e.g., rails, pheasants) may be only about 3 ft (1 m) high. In cold climates the aviary is usually enclosed and heated. Most aviculturists prefer to place birds in natural, planted surroundings. Many aviaries are maintained for pleasure by private aviculturists; others, especially large ones, are found in zoos or research institutions.