Ruby-throated and Black-chinned Hummingbirds are very close relatives and very difficult to tell apart in the wild. Well, I'd better qualify that. If you happen to get an adult male Ruby-throat and an adult male Black-chinned side by side in just the right light, then you would have no trouble telling them apart. The Black-chinned's throat is actually purple if you see it with the light shining on it, and the Ruby-throat's is, well, ruby-red. But it is not often that you will have such an ideal situation in which to identify your hummers. It is more likely that you will have the situation I was faced with today.
There was a flock of the little birds darting around me as I walked into my backyard. As best I could count, there were about ten. The rule of thumb for birders in our area is that every hummingbird that we see is a Ruby-throat until proved otherwise, but it has been an unusual migration season so far and I know I've had at least one Rufous Hummingbird in my yard which I reported on here last week. I know that Black-chinned Hummingbirds have also been reported in the area, although I'm always suspicious of such reports simply because it is so difficult to distinguish them from Ruby-throats. Nevertheless, I have been on the lookout for a Black-chinned in my yard. Today I think I might have found one.
Again, as with the Rufous, rather than the distinctively colored male, I have the more ambiguously colored female. The thing which drew my attention to her was not her coloring but her behavior. She acted different than the other birds. Specifically, I noticed that she was often pumping her tail as she flew about, and even after she perched, in a way that is not common with Ruby-throats. Then I took a closer look at her as she perched and began to notice some subtle differences.
The voice of the bird is no help in identifying her as the voices of the BCH and RTH are virtually identical.
So, do I have a Black-chinned Hummingbird in my yard? I'd be a lot happier and a lot more definite if I had an unmistakable adult male, but yes, I think I do. She's a female and the differences between her and the Ruby-throats in the yard are subtle, but when you see them together they are noticeable.
On the other hand, it wouldn't be the first time I tried to see something that wasn't there.