Because of the bright coloration, the identification of male hummingbirds is generally not difficult. However the general lack of coloration of the females makes their identification more difficult. All have green back feathers. One of the features that distinguishes two major groups of female hummingbirds is the presence or absence of reddish coloration of feathers on the sides and abdomen. Thus, one set is termed the Green and Grey species and the other set the Green and Rufous species. Both have two major genus. These are listed as follows:

Green and Grey species

Genus – Archilochus Small with medium length straightish black bills. Inner six wing primaries are narrower than the outer four. In the males the tails are forked and the gorgets (colored bibs) are shield shaped and ruby-red to bluish-violet in color. The chins are black.
The females have mostly plain under parts.
Both species have a white post-auricular spot. When perched the wings < tail.

Ruby-throated Hummingbird
Archilochus colubris
Black-chinned Hummingbird
Archilochus alexandri

Genus – Calypte Small with medium length straightish black bills. The inner primaries are the same width as the outer primaries. In the males the gorget and crown is rose-pink to violet in color and have ‘tails” at the corners. Greyish to whitish underparts.Wings <= tail. (In males wingtips fall well short of tail, i.e. wings < tail).

Anna’s Hummingbird Calypte anna
Costa’s Hummingbird
Calypte costae

Green and Rufous species

Genus – Selasphorus Small with medium length straightish black bills.Strong wing buzz in flight. Females have a rufus coloration of the underparts and tails. At rest wing tips < tail tip.

Broad-tailed Hummingbird Selasphorus platycercus
Rufous Hummingbird Selasphorus rufus
Allan’s Hummingbird
Selasphorus sasin

Genus – Stellula Small, short bill, short tail. Wing primaries are broad throughout. wings > tail. Males have a unique stripped gorget.

Calliope Hummingbird Stellula calliope

The above was adapted from Hummingbirds of North America The Photographic Guide
by Steve N.G.Howell 2002. Princeton University Press, Princeton, NJ.