The calliope is the smallest breeding bird found in Canada and the United States.[3][4] An adult calliope hummingbird can measure 7–10 cm (2.8–3.9 in) in length, span 11 cm (4.3 in) across the wings and weigh 2 to 3 g (0.071 to 0.106 oz).[4]
Calliope hummingbirds have glossy green on the back and crown with white underparts.[3] The adult male has wine-red streaks on the gorget, green flanks, and a dark tail.[3] When an adult male is competing for territory or females, the dark red feathers of its gorget distend outwards to create a more distinct appearance.[4][6] Females and immatures have a pinkish wash on the flanks, dark streaks on the throat and a dark tail with white tips. The only similar birds are the rufous hummingbird and the Allen's hummingbird, but these birds are larger with more distinct and contrasting rufous markings on the tail and flanks, and longer central tail feathers.
Female feeding insects to chicks
Habitat and distribution
The breeding habitat of calliope hummingbird is varied among open shrub habitats and altitudes. Nesting usually occurs at higher altitudes in the Rocky Mountains. Nests have been observed from as low as 600 ft (180 m) in Oregon and Washington elevation to the tree line at over 11,000 ft (3,400 m).[4] In Montana, the minimum elevation observed for breeding is 1,200 m (3,900 ft).[3][4] Open montane forest, mountain meadows, and willow and alderthickets may variously serve as breeding grounds. During migration and winter, they also occur in chaparral, lowland brushy areas, deserts and semi-desert regions. They nest in western North America from southern British Columbia and Alberta south to Colorado and southern California. During spring and summer, they move, mainly through Arizona and New Mexico and northern Mexico, to winter in southwestern Mexico as well as in Guatemala and Belize.[citation needed]
Behavior
Calliope hummingbirds are a migratory bird, generally leaving their breeding grounds earlier than most birds (although not as early as the rufous hummingbird) to take advantage of the late-summer wildflowers in the mountains of western North America. They are believed to be the smallest-bodied long distance migrant in the world.[4]
Adult males usually arrive on the breeding ground before females, from mid-April to early May. The male claims and vigorously defends a nesting territory in which he will breed with many females. The male takes no part in raising the young and often actually vacates the breeding grounds by the time the young hatch. The female usually builds an open cup nest in a conifer tree under an overhanging branch, though apple and alder trees have also been used. The nest is often built on the base of large pine cones and somewhat resembles a pine cone itself.[4] A nest may be used repeatedly over the course of several years. Two eggs are laid from late May to early July and are incubated for 15 to 16 days.[3] The young are capable of flight about 20 days after hatching.[4]
A group of three calliope hummingbirds hovering at a feeder to create the "humming" sound from their rapid wingbeats, while chirping by vocalization
The prominent humming sound of a calliope hummingbird – as for all hummingbirds during flight and hovering – derives from its rapid wingbeats while feeding or interacting with other hummingbirds.[7] Humming serves as an audible communication to alert other birds of the arrival of a fellow forager or potential mate.[7] The humming sound derives from aerodynamic forces generated by both the downstrokes and upstrokes of the rapid wingbeats, producing acoustic oscillations and harmonics.[7]
Courtship display and sonation
During courtship, a male calliope hummingbird hovers at accelerated wingbeat frequency up to 95 flaps per second (42% higher than normal hovering), creating a loud buzzing sound, with throat feathers protruding and facing a female.[3][8] The male then ascends temporarily to 20 meters and dives at high speed, with the rapid descent causing sonation of wing and tail feathers combined with vocalization, intending to attract attention of the female.[8] Research in a wind tunnel demonstrated that the male courtship display includes
sounds produced by three independent feather or vocal components, each with different acoustic characteristics, thus potentially containing different messages appealing to the female.[8] The tail feathers flutter at high frequency and hit each other to produce the buzzing sound.[9]
A hovering male calliope
Temperature regulation
The high metabolic rate of calliope hummingbirds – especially during rapid forward flight and hovering – produces increased body heat that requires specialized mechanisms of temperature regulation for dissipating heat, which becomes an even greater challenge in hot, humid climates.[10] As the smallest North American hummingbird species, calliopes appear to adapt their relatively higher surface-to-volume ratio to improve convective cooling from air movement by the wings.[10] When air temperatures rise above 36 °C (97 °F), thermal gradients driving heat passively by convective dissipation from around the eyes, shoulders, and feet are reduced or eliminated, requiring heat dissipation mainly by evaporation and exhalation.[10]
All hummingbirds dissipate heat partially by exhaled air, and from body structures with thin or no feather covering, such as around the eyes, shoulders, under the wings (patagia), and feet.[11][12] While hovering, calliope hummingbirds do not benefit from the heat loss by air convection during forward flight, except for air movement generated by their rapid wing-beat, possibly aiding convective heat loss from the extended feet.[10][13]
Status
As of 2019, calliope hummingbirds have a relatively stable breeding population of about 4.5 million.[4] As this species has a restricted wintering range in Mexico and travels an exceptional migratory distance annually of some 5,000 miles (8,000 km), it is on a watchlist for factors that could diminish the population.[4]
Threats
The calliope hummingbird does not have many predators. Its biggest threat is likely long cold spells since they reduce the volume of insects and nectar available in its environment.[14] The calliope hummingbird has been frequently observed making enemies with other birds, including Wright Flycatchers, Western Robins, and Red-tailed Hawks.[15] It is not afraid to dive at and attack birds that are many times larger than itself.
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