The White-collared Swift is a large swift; in many parts of its geographic distribution, in fact, it is the largest species of swift. The overall color is sooty black, except for a broad band of white feathers circling the neck. There is no discernable sexual dimorphism.
In most parts of its wide geographic distribution, the White-collared Swift is the largest species of swift, and easily is identifiable by the combination of very large size and the white collar. At both the northern and the southern extremes of its distribution, however, the White-collared Swift overlaps with a species that is similar in size and plumage:
In northwestern Mexico, White-collared Swift overlaps with the White-naped Swift (Streptoprocne semicollaris), which is slightly larger than White-collared. White-naped Swift has a black breast, the white being confined to a broad white collar across the back of the neck. Since the white breast band can be indistinct on juvenile White-collared, however, a difference in tail shape between the two species also is an important distinguishing character: the tail of White-naped Swift is square-tipped, lacking the slight fork or notch of the tail tip of White-collared (Howell and Webb 1995).
In eastern South America, the White-collared Swift overlaps with Biscutate Swift (Streptoprocne biscutata). The white collar of the Biscutate Swift is broken on the sides of the neck, although this feature can be difficult to discern in the field. Additionally, the white patch on the breast of Biscutate is roughly triangular in shape and the apex of the triangle points towards the bird's chin. In contrast, the white on the breast of White-collared Swift has a more ragged or wavy shape, is slightly narrower, and if anything forms a point in a downward direction. Furthermore, the Biscutate Swift has a paler (grayer) forehead, lores, and chin, and the tail is more square-tipped (Kirwan 2007).
Adult: Sexes similar. Crown black, usually slightly more sooty on the forecrown. Sides of head, chin and throat sooty black or very dark sooty. A conspicuous white collar encircles the neck; this collar is broadest on the breast. Rest of plumage black; the feathers of the back, scapulars, rump and uppertail coverts slight glossed with bluish, the feathers of the wings and tail more grayish black (paler, more sooty grayish, on the inner webs of remiges and rectrices). (Description based on Ridgway 1911.)
Immature: Similar to adult, "but white collar more or less broken by larger dusky central areas to feathers (especially on chest), and (in younger specimens) feathers of chin and upper throat streaked, those of lower throat tipped with whitish, and those of breast and abdomen and marginal under wing-coverts terminally margined with whitish" (Ridgway 1911: 700).
Juvenile: "Duller, sootier black than ... the adult. The feathers of the throat, face, crown, marginal coverts, primaries, secondaries and abdomen have soft, grayish-white tips; those of the breast and sides have broader, whiter tips; and those of the nape have very broad, sharply defined white tips" (Marín and Stiles 1992).
No information available.
Bare part color descriptions are from Ridgway (1911) and Wetmore (1968):
Iris: Dark brown
Tarsi and toes: Black to dusky
Total length: 20 cm (Fjeldså and Krabbe 1990); 20.3 cm (Hilty 2003); 20-21.5 cm (Howell and Webb 1995); Wingspan: 45-55 cm (Fjeldså and Krabbe 1990)
Linear measurements (from Ridgway 1911):
male (n=16), Streptoprocne zonaris mexicana:
Wing: mean 206.4 mm, range 200-215 mm
Tail: mean 75.6 mm, range 66.5-81.5 mm
Exposed culmen: mean 9.6 mm, range 8.5-10.5 mm
Tarsus: mean 22.3 mm, range 20.5-24.0 mm
female (n=14), Streptoprocne zonaris mexicana
Wing: mean 200.5 mm, range 194.5-206.5 mm
Tail: mean 72.0 mm, range 62.0-79.0 mm
Exposed culmen: mean 9.8 mm, range 9.0-10.5 mm
Tarsus: mean 22.0 mm, range 20.5-24.0 mm
male (n=10), Streptoprocne zonaris albicincta:
Wing: mean 196.5 mm, range 188.0-205.0 mm
Tail: mean 67.5 mm, range 62.5-72.0 mm
Exposed culmen: mean 9.5 mm, range 9.0-10.0 mm
Tarsus: mean 20.9 mm, range 19.0-22.0 mm
female (n=8), Streptoprocne zonaris albicincta:
Wing: mean 197.6 mm, range 185.5-207.0 mm
Tail: mean 69.4 mm, range 63.0-75.0 mm
Exposed culmen: mean 9.9 mm, range 9.0-10.5 mm
Tarsus: mean 21.1 mm, range 19.5-22.0 mm
Mass: mean 109.4 g, range 92.1-125.0 g, n=10, Costa Rica (Marín and Stiles 1992).