Meet the Calliope Hummingbird: Smallest Bird in the U.S.

Calliope hummingbirds are tiny, about 3 inches long and weighing around a pingpong ball. Eric Rasmussen, avian scientist at the MPG Ranch in Montana's Bitterroot Valley, says this hummingbird is the tiniest in the US.

This little species can be identified by many clues. Males and females have glossy, bright green heads and backs and white underparts. How to distinguish male and female hummingbirds.

Male Calliopes display a glittery gorget to attract mates. Females and youngsters have cinnamon under the gorget, which has magenta throat stripes.

Examine a Calliope's tail and wings. Eric claims their wingtips protrude slightly below the tail tip when perched.

These little hummingbirds live in Utah, western Wyoming, Montana, southern Canada, and those west of the Rockies. This species migrates to central Mexico for the colder months.

Calliopes migrate up to 2,800 miles north through the interior West in early spring, focusing on flowers. As Rocky Mountain wildflower meadows bloom, Calliopes fly uphill to mate and rear their young before following the mountains south in fall.