Its green back and iridescent purple head and throat distinguish the male, who has a slumped look and a stubby tail. “This small hummingbird has an all-green back without rufous (brown) coloration.
Underside is light gray on flanks to white on belly, and females have a white chin. Christopher Clark, assistant professor of biology at UC Riverside, says adult males have a vivid purple throat and crown.
Christopher claims the Costa's is sometimes confused with the larger Anna's hummingbird. Male Annas have a magenta throat and crown, not purple, and their bellies are grayer than white. Without a careful look, it's hard to tell if a bird is Costa's or Anna's.
The green and white females lack the males' strong purple throat and head feathers but seem similar. Identify them by a gray cheek patch and white eyebrow.
Juvenile Birding specialists Kenn and Kimberly Kaufman explain, “Young male hummingbirds are tricky to identify, because they’re often somewhere between the appearance of a female and an adult male.”
Reader Steve Dummermuth Jr. says, “I took this photo near Scottsdale, Arizona. I suspect a black-chinned or Anna's immature male.
Kenn and Kimberly remark, “For several reasons, we think this is a young male Costa’s hummingbird. Costa's black throat patch outline and pinkish purple lower throat patch are normal at this time. The breast and sides are clear whitish, but most Anna's and black-chinneds have gray-green sides.