Allen's hummingbird is a small bird, with mature adults reaching only 3 to 3.5 in (76 to 89 mm) in length and weighing approximately 2 to 4 grams in weight.

[3] They are known for their vibrant colors and energetic behaviors, making them a popular subject for birdwatchers and nature enthusiasts.

[3][5] Allen's hummingbird was formally described by the French naturalist René Lesson in 1829 and given the binomial name Ornismya sasin.

[9][10] The common name commemorates Charles Andrew Allen, an American collector and taxidermist who identified the bird in 1879 in Nicasio, California.

[14] The migratory habits of Allen's hummingbirds are closely linked to the availability of food sources, which affects their breeding success and survival rates.

This difference in timing helps reduce competition for food and nesting sites between males and females during the breeding season.

[3] Allen's hummingbirds are increasingly threatened by human actions and habitat loss, an impact causing their population to decline by about 80% since the 1960s.

[3] The courtship flight of male Allen's hummingbirds is a frantic back-and-forth flight arc of about 25 ft (7.6 m) similar to the motion of a swinging pendulum, followed by a high-speed dive from about 100 ft (30 m) during which tail feathers emit a characteristic sharp flutter to further attract attention of the female.

[15] Females tend to reside in forests with large trees, such as oak-pine, douglas fir, and redwood, providing ample protection and nest material.