Allen's Hummingbird, Selasphorus sasin
The back and crown on male Allen's Hummingbirds are green, as shown above (also note the ant in
his bill!) The same areas on the similar male Rufous Hummingbird are rufous, sometimes with a few
green feathers. (A very few Rufous males show extensive green backs; an all- or mostly-rufous back is
diagnostic of Rufous.)
Female Allen's and Rufous Hummingbirds generally can't be distinguished in the field by structure or plumage, except in the rare case where (as in the picture below) the tail feathers are visible. The Allen's second tail feathers out from the center (R2) are lanceolate (symmetrically spear-shaped) as can be seen especially clearly on this bird's left side in this picture. The same feather on the Rufous is notched on one side. This holds for both sexes in the two species; see the male Allen's below. Female Allen's can also be identified when at the nest, by location; Allen's nest in coastal California, while Rufous only migrate through.
A male Allen's showing its tail feathers; the outer two, R4 and 5, shown here overlapping on the bird's right, are
narrower in Allen's of both sexes than in Rufous. Below, another male Allen's with tail feathers spread; in this photo
they are blurred, but R4 and 5 are separated, so that it can better be seen how narrow they both are.
The pictures on this page were all taken at the UC Santa Cruz Arboretum, where Ruby-cluster bushes in the Australia section attract Allen's Hummingbirds, and make this probably the best place anywhere to observe and photograph this species. The males show off the distinctive Allen's display flight -- a series of short back-and-forth horizontal shuttles, followed by a steep ascent and then a rapid dive with a short turn upward at the end, accompanied by a wing-generated whistle.
The hummingbird's remarkable tongue can be as long as its head; it allows the bird to gather nectar by a combination of scooping and sucking, first revealed in 2012 by high-speed video of a hummer's drinking action.