Anna's hummingbird

Anna's hummingbirds have the northernmost year-round range of any hummingbird. Birds have been recorded in Alaska as early as 1971, and resident in the Pacific Northwest since the 1960s, particularly increasing as a resident population during the early 21st century.[17][18] Scientists estimate that some birds overwinter and presumably breed at northern latitudes where food and shelter are available throughout winter, tolerating moderately cold winter temperatures.[17][18]

During cold temperatures, Anna's hummingbirds gradually gain weight during the day as they convert sugar to fat.[19][20]

While their range was originally limited to the chaparral of California and Baja California, it expanded northward to Oregon, Washington, and British Columbia, and east to Arizona in the 1960s and 70s.[17] This rapid expansion is attributed to the widespread planting of non-native species, such as eucalyptus, as well as the use of urban bird feeders, in combination with the species' natural tendency for extensive postbreeding dispersal.[7]

In the Pacific Northwest, the fastest growing populations occur in regions with breeding-season cold temperatures similar to those of its native range.[17] Northward expansion of Anna's hummingbird represents an ecological release associated with introduced plants, year-round nectar availability from feeders supplied by humans, milder winter temperatures possibly associated with climate change, and acclimation of the species to a winter climate cooler than its native region.[17][18] Although quantitative data are absent, it is likely that a sizable percentage of Anna's hummingbirds in the Pacific Northwest still do migrate south for winter, as of 2017.[18]

Female Anna's hummingbird in nocturnal torpor during winter; −8 °C (18 °F), near Vancouver, British Columbia. The bird remained in torpor with an unchanged position for more than 12 hours.

Anna's hummingbirds with inadequate stores of body fat or insufficient plumage are able to survive periods of subfreezing weather by lowering their metabolic rate and entering a state of torpor.[16][21]

When studied in colder temperatures at mountainous elevations, Anna's hummingbirds used torpor more frequently than at lower elevations.[16]

While collecting nectar, Anna's hummingbird assists in plant pollination.[12] There is evidence that Anna's hummingbirds in flight generate an electrostatic charge that adheres pollen to their beaks and feathers, facilitating transfer of pollen grains to hundreds of flowers per day while foraging for nectar.[22] This species sometimes consumes tree sap.[23]

Anna's hummingbirds can shake their bodies 55 times per second to shed rain while in flight, or in dry weather, to remove pollen or dirt from feathers.[24] Each twist lasts four-hundredths of a second and applies 34 times the force of gravity on the bird's head.[24]

During hovering flight, Anna's hummingbirds maintain high wingbeat frequencies accomplished by their large pectoral muscles via recruitment of motor units.[25] The pectoral muscles that power hummingbird flight are composed exclusively of fast glycolytic fibers that respond rapidly and are fatigue-resistant.[25]

Unlike most northern temperate hummingbirds, the male Anna's hummingbird sings during courtship. The song is thin and squeaky, interspersed with buzzes and chirps, and is drawn to over 10 seconds in duration. During the breeding season, males can be observed performing an aerial display dive over their territories. When a female flies onto a male's territory, the male rises up about 130 ft (40 m) before diving over the female. As the male approaches the bottom of the dive, it reaches an average speed of 27 m/s (89 ft/s), which is 385 body lengths per second.[26] At the bottom of the dive, the male travels 23 m/s (51 mph), and produces an audible sound produced by the tail feathers, described by some as an "explosive squeak".[27][28]

A female incubates eggs in a camouflaged nest.

Two nestlings are fed by a female hummingbird.

Open-wooded or shrubby areas and mountain meadows along the Pacific coast from British Columbia to Arizona make up C. anna's breeding habitat. The female raises the young without the assistance of the male. The female bird builds a nest in a shrub or tree, in vines, or attached to wires or other artificial substrates. The round, 3.8-to-5.1-centimetre (1.5 to 2.0 in) diameter nest is constructed of plant fibers, downy feathers and animal hair; the exterior is camouflaged with chips of lichen, plant debris, and occasionally urban detritus such as paint chips and cigarette paper.[13] The nest materials are bound together with spider silk. They are known to nest as early as mid-December and as late as June, depending on geographic location and climatic conditions.[7][29]

Anna's hummingbirds may hybridize with other species, especially the congeneric Costa's hummingbird.[13] These natural hybrids have been mistaken for new species. A bird, allegedly collected in Bolaños, Mexico, was described and named Selasphorus floresii (Gould, 1861), or Floresi's hummingbird. Several more specimens were collected in California over a long period, and the species was considered extremely rare.[30]

The specimens were the hybrid offspring of an Anna's hummingbird and an Allen's hummingbird. A single bird collected in Santa Barbara, California, was described and named Trochilus violajugulum (Jeffries, 1888), or violet-throated hummingbird.[31] It was later determined to be a hybrid between an Anna's hummingbird and a black-chinned hummingbird.[32][33]