Good question! Thirty years ago, field guides simply pronounced that females and young birds were "virtually indistinguishable" in the field, and left it at that. Thanks to a lot more study, we know that's not strictly true.
The green body ("contour") feathers of immature Ruby-throats, when they first come in, have a buffy edge to them, which gives the bird an overall golden-green appearance. You can see the edge if you are close enough and/or have good optics. Because those first body feathers will last the bird into its first winter, they are a good way to distinguish adult females from immature birds of either sex (assuming you can see the bird's back clearly).
In the hand, with a jeweler's loupe, it's even easier; at hatch time, the covering on the bill of a hummingbird has tiny corrugations, or wrinkles, which gradually smooth out over the course of its first year of life. Birds examined shortly after they first take flight may have 90% of their bill covered with these corrugations; a young bird in late winter may only have 20-30% still covered, and in early spring, that could be as low as 10% - or none. But this isn't a useful field technique, since it requires having the bird "in hand".
Female Ruby-throats rarely have much, if any, stippling in the gorget. On occasion you may see some very tiny, fine lines there, but generally speaking, stippling in the throat suggests a male bird. And of course, many immature male Ruby-throats have one or more colored feathers already developed in the gorget by this time, which is a sure sign: under any normal circumstances, a female Ruby-throat will have no colored gorget feathers. (I say "normal circumstances" because there's almost certainly some rare exception to this rule, such as a bird with a hormonal disturbance that caused an abnormal plumage.)
As with the bill corrugations, there's a way to determine sex for a bird in the hand as well. There's a difference in shape for the sixth primary (primaries are the outermost wing feathers, the ones which make up the biggest portion of the wing surface). However, this is something only seen in Ruby-throats, so it's not a universal "hummingbird sex" test. And again, it's something you can't see without looking at the bird's spread wing in hand.
So: buffy-gold tips to the green body feathers: immature bird.
Heavily stippled throat or few colored gorget feathers: immature male.
Really clear throat: probably female, though it could still be a male.
Don't ever feel bad about not being able to identify every bird. I can't. Many people far more experienced than I am can't. An ornithology professor I know likes to tell the story of how he identified a Snowy Owl (using a good-quality scope and studying it carefully), only to find out it was a branch on a tree.
It'll come with practice. Having a feeder close to your windows can help, because the birds will be right there in front of you, to see things like the buffy feather edges and the gorget feathers. Most of all, enjoy them!
Kevin Morgan
Baton Rouge, LA
This message has been edited by CowboyinBRLA on Sep 7, 2009 5:44 PM
Along these lines of trying to tell females from juvies: Do females molt after nesting? During late August I see hummers come to the feeder that are lovely and sleek -- many showing juvenile behavior, and others that are 'shaggier' and appear to have dark patches along the breast and neck area, not stippling but darker areas. I haven't been able to figure it out. I guess I could try to find photos of molting birds and compare.
Yes, adult Ruby-throats of both sexes molt all their body feathers just after completing the breeding season. Adult females will often have a "nest mark" (that tattered line across the breast) for the first few weeks after nesting is over, but those feathers may be largely replaced by the time they start making their way south. Nancy may be able to address this further, but I think the post-breeding molt generally happens quickly and is mostly over by the time the birds start to move.
Young birds will probably show a less intense nest mark, but they'll often carry it into the fall and winter, when they being to molt their contour feathers for their first "adult" plumage. Of course, some of the feathers may be replaced along the way, during the fall, but Ruby-throats appear to undergo a pre-Spring migration molt that freshens up the body feathers and puts them in good shape for the breeding game.
