White-throated swift

White-throated swifts are year-round residents of coastal and southern California, southern Arizona, southern New Mexico, western Texas, central Mexico, and the Sierra Madre Occidental and Oriental mountains.[2] Migratory breeding populations can be found from Arizona and New Mexico north to southern British Columbia, and from central California east to eastern Colorado and Wyoming.[2][6] In the southern part of its migratory range, birds arrive in mid-March and depart in mid-October, while in the northern portion, they do not arrive until mid-April to early-May and depart in late-August to mid-September.[2][7] There are two main areas in which non-breeding wintering populations can be found: the first includes southwestern California, eastern Arizona, the Texas Panhandle, while the second is in El Salvador, Guatemala, Honduras, and the Mexican state of Chiapas.[2]

White-throated swifts flying near cliff habitat.

Swifts roost in cliffs, bluffs, and canyons, as well as man-made structures like bridges, overpasses, and walls of quarries.[2] In natural settings, swifts form colonies of as many as 400–500 birds in protected cliff crevices that are generally 6–50 meters above the ground.[2][6][8] White-throated swifts will sometimes roost and nest in amongst colonies of swallows, including cliff, violet-green, and northern rough-winged swallows.[9][10] While foraging, white-throated swifts are found flying over meadows, agricultural fields, and open areas along the edges of ridges and hills.[2]

Swifts are aerial insectivores who frequently forage in areas of rising air along the edges of canyons, foothills, or mountains to capture insects.[2] White-throated swifts have also been observed foraging over agricultural lands, and are known to follow harvesting machinery to capture insects disturbed by the equipment.[12] Stomach content analysis has found that beetles, flies, bees, and true bugs (Hemiptera) constitute a majority of the white-throated swift's diet.[9][13] Adult swifts nourish their nestlings by carrying a bolus of arthropods in their buccal cavity and feeding it to the young.[3] Swifts derive most of their water from their diet but can also be skimming the surface of ponds to drink.[7]

Known predators of White-throated swifts include peregrine falcons, prairie falcons, and American kestrels.[14][15][16] When predators are detected, swifts are known to quickly lower their altitude and fly just above the vegetation.[2] There is no data available on levels of nest predation.[2]

White-throated swifts make vocalizations while in the air that are described as "staccato chattering",[3] and may also make a sharp one- or two-note call[2] or a shrill drawn-out "scree" call.[17] In the morning and evening, "twittering" sounds may be made at their roost sites.[8] Additionally, juveniles may make begging sounds that are higher-pitched versions of the adult's shrill scree call.[2]

During the courtship season, white-throated swifts may perform gliding displays and plummeting courtship falls prior to forming monogamous pairs.[2] Once a pair is established, nests made of plant material and feathers held together with saliva are built on rocky cliff faces or human-made structures.[18] These nests are usually 8–10 centimeters across, and 2.5–2.8 centimeters deep, and may be used for several years in a row.[18] An average of 4–5 white matte eggs are laid over the course of 4–6 days, with incubation generally beginning only after the last egg is laid.[3] In southern parts of their range, egg-laying occurs in April, but in more northerly areas, clutches are usually laid throughout May, with a median first-egg date of May 13.[2] The incubation period lasts on average 24 days, with swifts weighing approximately 2 grams upon hatching.[3] Both parents feed the nestlings, and by the time of fledging, swifts may weigh up to 46 grams.[2] There are no documented cases of swifts attempting a second brood if the first nest fails.[2]