Biscutate Swift Streptoprocne biscutata
Version: 1.0 — Published September 19, 2014
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Biscutate Swift is a very large, blackish brown swift species, with white patches on the nape and on the breast, forming a broken white collar around its neck. The tail is blunt tipped. Males and females are similar.
Biscutate Swift easily is confused with the more widespread White-collared Swift (Streptoprocne zonaris), with which it can form mixed-species flocks. The single best feature used to separate the two is the white collar, which on the Biscutate Swift is broken and on the White-collared Swift is a complete neckband. It can be difficult in the field to determine if the collar is complete or broken, however, especially on birds that are distant. A further distinction is that the collar is broader on the breast of Biscutate Swift than in White-collared, and furthermore the breast path of Biscutate is slightly rhomboid or triangular in shape, pointing up towards the chin. On White-collared Swift, this collar is set slightly forward from the wings, whereas on the Biscutate Swift, the lower edge of the collar always appears to fall at the level of or below the leading wing edge. The head of White-collared Swifts is uniformly colored, whereas Biscutate Swifts have a paler gray forecrown, chin, and lores. The tail of Biscutate Swift is slightly more squared off than the notched tail of White-collared, although both species often fly with their tail closed (Kirwan 2007).
White-naped Swift (Streptoprocne semicollaris) also is similar in appearance to Biscutate Swift, but its white patch is confined to the nape. The ranges of these swifts also do not overlap, as White-naped Swift is only found in western Mexico.
The following description is based on Chantler (2000), and refers to nominate biscutata; see also Geographic Variation:
Adult: Sexes similar. Head sooty brown, but distinctly paler on the forecrown, chin, lores, and a line of feathering above the eye. Body mostly sooty blackish brown (with some pale grayish white fringing in fresh plumage), except for a broken white collar around the neck: this collar extends across the nape, to just behind the auriculars; and also forming a broad band across the breast, this patch slightly rhomboid or triangular in shape, with the acute end pointing forward, towards the chin. Rectrices and remiges blackish on the outer webs, pale gray brown on the inner webs. The undersurface of the remiges are similar in color to the greater underwing coverts, but the median and lesser underwing coverts are blacker.
Juvenile: In fresh plumage, fringed with grayish white, most noticeably on the belly and on the tips to the rectrices.
Little published information on adult molts. Nestlings are naked upon hatching, and by 13 days will have grown a first feather coat of gray semiplumes. Juvenile plumage is fully established by 34 days (Pichorim 2002).
Tarsus and toes: black
Bare parts color data from Belton (1984).
Total length: 22 cm (Chantler 2000)
Linear measurements (from live birds; Pichorim 2010):
wing length: nean 206.3 ± 5.2 mm (n = 812)
tail length: mean 69.9 ± 4.6 mm (n = 802)
tarsus length: mean 25.9 ± 0.6 mm (n= 695)
bill length (exposed culmen): mean 9.94 ± 0.36 mm (n = 658)
Wing and tail length vary seasonally, due to feather wear; lengths of the wing and tail are greatest when the feathers are fresh, and are shorter at the beginning of the molt (Pichorim 2010).
Mass: Varies with age, time of day, and season. Mean mass of birds captured at dawn: 115.5 ± 9.9 g (n = 1320) (Pichorim 2010). Birds captured later in day are heavier, presumably due to the mass of the insects consumed during the day; the overnight descrease in mass is up to 13% (Pichorim 2010). In adults, mass peaks in spring and in fall; decreases in mass are correlated with molt (austral winter) and with breeding (asustal summer) (Pichorim 2010).