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.
They are hatched in a cold climate, they spend nights on nesting grounds where the temperatures are near freezing. They migrate down mountain corridors where the temperatures are cold. Finally, these U.S. Rufous are continually being refined by the genes of cold hardy ancestors that have endured severe winters. The presence of Rufous going about their daily routine in times of severe cold requires rethinking our impression of hummingbirds in general. Unfortunately, in many cases much of the information about Rufous available to the public is both incomplete and inaccurate.