Size: Allen's is a small hummingbird (3.5 inches) with medium short all black straight bill.
Distinguishing Characteristics: The Allen's male can be distinguished from the Rufous by its green back and crown. The female Allen's and Rufous cannot be distinguished in the field. The R5 tail feather (rectrix) is slightly narrower in the Allen's (on both male and female). You can see this in the last photo above.
Habitat: Moist coast and riparian forest and woodland, typically only to 1,000 feet (Arizona the obvious exception).
Range: Allen's has a very small breeding range which was restricted from the Northern California to Southern Oregon coast. It's range has been expanding into Southern California over the past few years. It is occasionally found inland but not when breeding. Allen's sometimes finds its way into the Madrean Sky Island foothills of Southern Arizona. It's winter range is also quite small being restricted to forest edges in Southern Mexico.
Seasons: In Arizona, it's found primarily in July, August, and September. It's thought that these are migratory birds headed to their wintering location. Allen's winters in a relatively small area in south-central Mexico.
Where to Find: Allen's is rare to Arizona. The first two photos are from Ash Canyon (in the foothills of the Huachuca Mountains). Ash Canyon B&B has closed but try Ramsey Canyon. The last photo was from WOW Arizona a privately owned residence near the town of Catalina on the northside of the Catalina Mountains (again in the foothills). Allen's have also been reported in Madera Canyon and Patagonia. Keep an eye on the Audubon Society's Rare Bird Alert and the ABA's Birding News for Arizona and New Mexico.
Comments: Allen's is typically less aggressive than the Rufous. When found in Arizona there are often Rufous around as well. This makes identifying them a little more difficult -- especially the females.