Five varieties of hummingbirds are presented as stamp designs produced in booklet form. Hummingbirds often are referred to as the helicopters of the avian world. They have evolved a unique form of flight. While other birds can hover for a moment or two, the darting, start-stop, up -down, forward-backward flight of "hummers" belongs to them alone. Hummingbirds scull the air rather than stroke it, gaining constant lift from what is more of a forward-and-backward movement than the more usual up-and-down flapping of other birds. When hovering, their bodies assume a nearly vertical position. To power such an extraordinary set of wings, which never have a rest when the bird is in flight, hummingbirds have the largest pectoral muscles for their size of any animal alive. On a cloudy day, hummingbirds are inconspicuous. Given a ray of sunshine, their feathers spring to life in glinting, iridescent colors that have no match in nature.