Chestnut-collared Swift is a medium sized swift (but is one of the smallest species of Streptoprocne), with a broad, only slightly notched tail, rather broad wings, and blackish plumage. The adult male has a distinctive broad rufous collar, a feature shared only by Tepui Swift (Streptoprocne phelpsi). Females usually lack the collar, or the collar is greatly reduced in extent or is duller, but a few females may develop a collar as extensive as in the maler. Juveniles are paler than adults, and have only a dull, partial collar.
Throughout most of its range, Chestnut-collared Swift is the only species of swift with a rufous collar, and so the male easily is distinguishable when seen well in good light. Most females lack the rufous collar, however, and the collar often is not visible on the male under field conditions (e.g., at a distance, in low light, when backlit, etc.). Size and shape then are useful features that aid in identification. Chestnut-collared Swift is slightly larger than Chaetura swifts, with blacker and more uniform body plumage, broader wings, and a slightly notched tail. Female Chestnut-collared Swift can be confused with species of Cypseloides, such as Black Swift (C. niger), White-fronted Swift (C. storeri), White-chested Swift (C. lemosi), White-chinned Swift (C. cryptus), and Spot-fronted Swift (C. cherriei), but female Chestnut-collared can be distinguished from all by its distinctive buzzy vocalizations.
Chestnut-collared Swift is most similar to Tepui Swift (Streptoprocne phelpsi). These species usually do not overlap, as Tepui Swift primarily occurs in the pantepui region of northern South America, where Chestnut-collared does not occur; but Tepui Swift is a vagrant or rare disperser to the range of Chestnut-collared in the coastal cordilleras of northern Venezuela. The most noticeable difference between the two is the shape of the tail: Chestnut-collared has a shorter, less deeply notched tail. Other differences (which may be more difficult to perceive under field conditions, but are readily apparent in the hand) are: Chestnut-collared Swift is slightly browner, less blackish; has shorter wings; its collar is deeper rufous (less orangey, as in Tepui); the collar is reduced or absent on female Chestnut-collared (but is almost as prominent on female Tepui as on the male); and the collar of Chestnut-collared does not extend onto the chin and upper throat, as it does on Tepui Swift. These two species also have different vocalizations.
The following description is based on Marín and Stiles (1992) and on Chantler (2000), and refers to nominate rutila; see also Geographic Variation:
Adult male: Crown sooty blackish brown; forecrown and a narrow line of feathers over the eye have narrow grayish brown fringes. Narrow black patch around the eye. Chin to upper throat and auriculars grayish brown, with a variable number of dull rufous feathers mixed in. Lower throat down to the upper breast rufous, extending up around the sides of the neck to the nape, forming a broad collar. Back blackish brown; upperparts generally quite uniform, the rump and uppertail coverts only a little paler. Rectrices sooty blackish brown. Lesser wing coverts slightly paler than the back. Generally, the outer primaries are the blackest portion of the upper surface of the wing, and the secondaries paler, but the difference is slight. The underparts is a little paler than the upperparts, and is fairly uniform in color from the lower breast below the collar to the undertail coverts. When fresh, the feathers are narrowly fringed with grayish white, most prominently on the undertail coverts. Plumage becomes slightly browner when worn.
Adult female: Typically the collar is reduced or lacking, and the body plumage is paler than in males. A minority of females have a partial or even a complete collar; these females also have blacker plumage.
Juvenile: Either lacks the rufous collar or has only dull rufous tips to the feathers of the nape and sides of the throat, forming a partial collar. Narrow white fringing to the tips of the remiges and median wing coverts. Body generally brownish black, paler than adults.
There is little information on the molts of Chestnut-collared Swift; apparently follows the complex basic molt strategy. Adults have a single complete annual (basic) molt. Typically this follows the breeding season, but, at least on Trinidad, some individuals, especially those raising a second brood, may initiate molt while still incubating eggs or tending nestlings (Collins 1968). On Trinidad, molt extends from August to early December (Collins 1968).
The prebasic molt begins with the primaries, followed by the secondaries; body molt is initiated soon after the primaries begin to molt (Collins 1968).
Juveniles apparently have at least a partial body molt, but do not molt the remiges or rectrices in their first year.
Iris: brown, dark brown
Bare part color data from Stiles and Skutch (1989) and on specimens in the Field Museum of Natural History.
Total length: 12-13 cm (Howell and Webb 1995), 12.7-13.5 cm (Hilty 2003), 13 cm (Hilty and Brown 1986), 13-13.5 cm (Ridgely and Greenfield 2001b), 14 cm (Stiles and Skutch 1989)
Linear measurements (from Marín and Stiles 1992; live birds, sex undetermined):
wing length: mean 127.33 mm ± 3.15 mm (n = 131)
wing area: mean 42.67 cm ± 0.58 cm (n = 4)
tail length: mean 45.20 mm ± 1.81 mm (n = 136)
bill length (exposed culmen): mean 5.71 mm ± 0.25 mm (n = 131)
tarsus length: mean 11.25 mm ± 0.44 mm (n = 129)
Mass: mean 21.32 g ± 1.81 g (n = 139, sex undetermined; Marín and Stiles 1992)