Black Swift is the largest swift north of Mexico with black to dark brown plumage. The tail is long and slightly forked on males, possibly indicating age of the bird. Wings are long and scythe-like. Forehead, throat, lower abdomen, and undertail coverts have white tipping of varying amounts; feathers of the head are overall lighter sooty gray and/or brown. Males and females have only very slight plumage differences and distinguishing between sexes is difficult. This topic has been debated since 1882 with some on the side that the adult males and females are indistinguishable by plumage, while others state there are differences (Drew 1882; Hartert 1892; Ridgway 1911; Swarth 1912; Brooks 1924; Griscom 1924; Swarth 1924b). However, when compared to DNA sexing, adult males and females can be differentiated by plumage (K.M. Potter, personal observations). Juveniles have more white tips on feathers, especially on forehead, throat, lower abdomen, and undertail coverts.
Identification is often exceedingly challenging given that this species is often seen at great distances high in the sky; distinguishing it from all other Cypseloides swifts in Central and South America is very problematic. Sometimes differentiating Black Swift from other congeners is possible only with bird or specimen in hand.
Adult: Sooty black to dark brown. Long tail noticeably forked on older adult males, often fanned in flight. Differences between sexes include the forked tail of male and females having more white tipping on forehead, lower abdomen, and undertail coverts. Wings are scythe-like in shape. Males average larger than females but there is a wide range of overlap (K.M. Potter, J.P. Beason, C. Gunn personal observations).
Juvenile: Blacker overall than adults and white tipping more extensive on forehead, throat, lower abdomen, undertail coverts, and tips of remiges (K.M. Potter, J.P. Beason, C. Gunn personal observations).
Molt has never been witnessed on any adults banded in Colorado during northern hemisphere summer months. It is believed that molt occurs exclusively during winter months in South America. More study is needed. No specimens of C. niger costaricensis collected from April to late June in the gorge of the Río Tiribí, Costa Rica, showed an overlap between molt and breeding (Marín and Stiles 1992). Rathbun (1925) stated that in the 31 specimens he observed (collected in Washington state from June to September), plumage seemed to have been recently renewed, being fresh and bright and showing no traces of a molt.
The iris is dark brown in adults and nestlings. In adults and juveniles near fledging, the eyelids, bill, and nares are sooty black (no cere present in this species), as are the tarsi, feet and toenails.
In nestlings, the tarsi and feet are gray-pink, with sooty black nails. An egg tooth, when present, is very light gray to white (K.M. Potter, J.P. Beason, C. Gunn personal observations).
Across the three subspecies, there are general differences in plumage measurements and weight, with C. n. borealis being the largest subspecies, C. n. niger the smallest, and C. n. costaricensis approximately midway between these two extremes, but with some overlap among the subspecies. Individual techniques used for plumage measurements can introduce some variation in accuracy.
C. n. borealis wing chord measurements range from 152-175 mm, with mean male wing chord 166 mm and mean female wing chords 160-164 mm. Tail length measurements range from 37-67 mm, with male tails averaging 57-61 mm and female tails averaging 51-53 mm. Weights range from 39-57 g, with a mean of 46 g. Mass can vary greatly depending on annual cycles such as breeding and migration (Ridgway 1911, Pyle 1997, K. M. Potter unpublished data).
C. n. costaricensis have a mean wing chord measurement of 159 mm (both sexes included), a mean tail length measurement of 53.6 mm, and a mean weight of 35.7 g (Marin and Stiles 1992, Lowther and Collins 2002).
C. n. niger have wing chord ranges from 140-161 mm, with a mean wing chord measurements for males of 146 mm and 144.5 mm for females. Tail length measurements range from 51.5 to 69 mm, with mean tail length measurements for males 63 mm and 56 mm for females Griscom 1924, Eisenmann and Lehmann 1962, Lowther and Collins 2002).
Pyle (1997) states that the forked tail of males is greater on After Second Year birds than Second Year or Third Year birds. Of 17 males captured >2 years of age in Colorado, only eight have shown an increase in the amount of tail forking (length of r5 – r1). However, specific ages of swifts when initially captured are unknown. In total, 53 r5-r1 measurements of 30 males have varied from 4 to 15 mm in Colorado. Pyle (1997) also states that female r5-r1 values are 0-8 mm and males are 8-16 mm for After Hatch Year or After Second Year birds.