Courtesy Kenn Kaufman
The normal range north of Mexico is limited to canyons in a few mountains near the U. S. border, and along rare, perennially flowing streams in the shady mountain canyons of southeastern Arizona, southwestern New Mexico, and western Texas.
This northern hummingbird is one of the most ecologically selective, preferring the edges of mountaintop conifer forests in the highlands of Mexico and the shady understory of deciduous streamside forests in the mountain ranges of the southwestern United States. This hummingbird's breeding range consists of moist pine-fir and highland deciduous forests, pine-oak woodland, forest edges, second growth, and shrubby areas. Though they are usually found in moist, shady areas, Blue-throated Hummingbirds may wander into drier habitats nearby when foraging. They can also occupy appropriate habitat over a broad elevational range. Migration routes and winter ranges include lower, often drier habitats. The winter range also includes tropical deciduous and evergreen forests to elevations as low as sea level.
Blue-throated Hummingbirds feed on floral nectar and small invertebrates, including flying insects and small spiders; they may raid spider webs for prey spiders. The birds hover at flowers to drink nectar; nectar-feeding frequency is highest early and late in the day and during inclement weather. These hummingbirds pursue flying insects from exposed perches and in sustained foraging flights over forest openings and water bodies. They also glean vegetation for invertebrates, flying up and down tree trunks and large branches and hovering to pluck insects from bark and foliage, particularly during periods of low invertebrate activity. The large size and aggressive nature of this species make it a regular at both natural nectar sources and feeding stations. Artificial feeding may help northern members of this species survive winters in high-elevation habitats; a few individuals are now year-round residents at feeding stations in Arizona, far from the known northernmost limit of the species' permanent range in northwestern Mexico.
In Arizona, adult Blue-throated Hummingbirds return to their breeding areas in March, and nest-building begins by early April. In high elevations of southern Mexico, the season may begin up to two months later. Nest-building immediately precedes mating and egg-laying. Nest sites are typically sheltered by overhanging rocks or stream banks; nests may be built on tree branches, roots, leaves, or stems, on sheltered rock ledges, at cave entrances, in abandoned Black Phoebe nests, or attached to human-made structures. The hummingbirds' preference for protected sites makes their nests less vulnerable to weather and heat loss. The nest, primarily spider silk, is built entirely by the female. Other materials are added as lining and camouflage, including plant fibers, seed plumes, feathers, animal hair, insect cocoons, and fern hairs. Female may remove materials from old nests to use in new nests. Protected nest sites often persist for many seasons and are used as foundations for subsequent nests.
Females typically produce multiple broods. Two dull white eggs are typically laid one day apart after the nest cup is completed. Incubation may be delayed until both eggs are laid. The female incubates the eggs for 17 to 19 days, then begins her nearly continuous brooding of the featherless hatchlings as soon as they appear. Brooding decreasing in frequency and duration until nestlings are 10 to 12 days old and capable of thermoregulation. Nests visits by males are rare. The female feeds the young by regurgitation of predigested arthropods mixed with nectar. In Arizona, the young leave the nest 24 to 26 days after hatching; the female locates her fledgling by calls and feeds them for at least several days after departing the nest. More mature young actively follow and beg food from the female.
Blue-throated Hummingbirds are partial short-distance migrants. Northernmost populations are largely absent from their breeding range during winter; a few remain as residents, mostly near feeding stations. Migration in southern populations may be primarily altitudinal; populations on Cerro San Felipe, Oaxaca, retreat from higher elevations in winter, returning in late June. Other populations return to their northern breeding areas from by May. They typically leave their breeding areas in Arizona, Texas, and Sonora, Mexico beginning in September, migrating south mostly through higher mountain elevations.