I did not really think I could do a good job of this exercise, but slowing down and focusing on the details and proportions yielded a solid effort! Perhaps a little fat, but so are the birds at my birdfeeder :)
I studied two kinds of hummingbirds at the feeders in Barbados. It took me a while to discover that there were really two types, and then I became more aware of their difference in size and in their beak shape. One kind that comes rarely has a crest- but only on the male- so then I realized that for some varieties I could differentiate not only their type but sex.