The first molt of any feathers that we normally see is on the underparts. In very young wintering Rufous, the loosely textured feathers of the breast and belly are easily dislodged. The feathers of the wing molt symmetrically from the body sides outward. Normally, when we see a Rufous with all new wing feathers except the outside two on each side, they are subject to leave almost any time. Even though the brilliant gorget feathers will continue to erupt from their sheathing during the winter months, the back of a male Rufous will be essentially all-brown before the gorget color is complete.
Males tend to have a much harsher tone to all their vocalizations.
In addition, males are much more active vocally than females. Even as very young birds, males in winter begin to hone the intimidation skills that they will employ on their breeding grounds to dominate a breeding territory. Like other hummingbird species, young wintering Rufous males appear to do their version of a territorial morning song, in effect a declaration of territory. With experience, you can tell whether a bird is a male or female simply by their voice and how and when they use it. Rufous hummingbirds are very cold hardy.