The high-pitched metallic buzzing of a male Allen's Hummingbird's wings in flight heralds the species' midwinter arrival in coastal California during “spring” migration from central Mexico. Much squabbling ensues as the fiercely territorial males fight to defend food sources and breeding territories in anticipation of the arrival of females, soon to follow. To defend its territory, the male has one of the most complex and spectacular dive displays of any North American hummingbird. As a prelude to his dive, the male swings back and forth in symmetrical, pendulous arcs, slowing at the top of each arc to shake his body and emit a loud, cricket-like trill with the wings. He then ascends up to 30 meters before entering his power dive, pulling out of the dive just above the object of his display while emitting a loud, metallic shriek with his tail feathers. The object of the display may be an intruder in his territory or a female seeking a mate.
As with other hummingbird species, the female raises two young on her own and may raise two to five broods during a prolonged breeding season. Males begin departing the breeding grounds on southward migration while females are still raising young. The females depart next, to be followed eventually by the juveniles.
The overall breeding range of Allen's Hummingbird is small compared with other species of North American hummingbirds. Most hummingbirds breeding in the United States are represented by only one subspecies; nevertheless, within its narrow range, two subspecies of Allen's Hummingbird exist, Selasphorus sasin sasin and S. s. sedentarius. These races differ slightly in morphology but have very different migratory behavior, nominate sasin being a migrant wintering in central Mexico while sedentarius is nonmigratory. S. s. sedentarius is currently expanding its breeding range in southern California.
The seasonal movements of the migratory populations are early compared with most North American migrants. Northbound birds may depart on migration as early as December and arrive on the breeding grounds as early as January. Adult males may begin their southward migration in mid-May and arrive on non-breeding grounds as early as August.
The species was named for Charles Allen, a collector who lived in Marin Co., CA, in the late 1800s and early 1900s. Allen was among the first to note the differences between the Allen's Hummingbird and the closely related Rufous Hummingbird (Selasphorus rufus). In 1877 he sent specimens and notes about the two species to William Brewster in Massachusetts, who turned them over to Henry Henshaw, who described the species and named it Selasphorus alleni (Henshaw 1877).
Unbeknownst to them, the species had already been described by Lesson (1829), based on specimens collected in the bay area by Botta (Grinnell 1931). Lesson thought he was re-naming the Rufous Hummingbird, which back then was called the Nootka Hummingbird, as it was discovered at Nootka Sound on Captain Cook's third voyage (Swainson 1830). Lesson therefore gave it the name Ornismya sasin, in which sasin (or sasinne) means ‘hummingbird' in Nuu-chah-nulth, the language of the indigenous people to Nootka (Mavor 1813). Grinnell (1931) recognized Lesson's mistake, and Lesson's scientific name therefore takes priority. Much of the early literature on this species uses the scientific name given by Henshaw (Mearns and Mearns 1992a).
The natural history of Allen's Hummingbird was described in early studies by Aldrich (Aldrich 1938b, Aldrich 1945b, Aldrich 1956). Interspecific competition, especially with sympatric Anna's Hummingbird (Calypte anna), was the subject of studies by Pitelka (Pitelka 1951b) and Legg and Pitelka (Legg and Pitelka 1956). Ortiz-Crespo (Ortiz-Crespo 1969, Ortiz-Crespo 1980) studied interspecific competition and male territorial behavior.