SPECIES

Black Falcon Falco subniger Scientific name definitions

Stephen Debus
Version: 2.0 — Published March 17, 2023

Plumages, Molts, and Structure

Plumages

Black Falcon has ten full-length primaries numbered p1 (proximal) to p10 (distal); 13 secondaries numbered s1 (distal) to s13 (proximal); three tertials numbered t1 (proximal) to t3 (distal); and 12 rectrices numbered r1 (central) to r6 (outer) on each side of the tail. The wing is diastataxic, with a slight gap between s4 and s5. The wings are long, broad, and sharply pointed at the tips, with p9 longest and slightly longer than p8; p9 and p8 (slightly) are emarginated on the outer web, and p9 and p10 are emarginated on the inner web. Rectrices r1 to r5 are almost equal in length, creating a square to slightly rounded tail tip; r6 is noticeably shorter than r5, creating a "stepped" effect on the outer edges of the tail.

Plumages, molt, and age criteria are taken from Marchant and Higgins (1), Debus (3), Debus et al. (7), Debus and Olsen (5), Debus and Zuccon (8), Charley et al. (9), Whelan et al. (10), Schoeb et al. (11), and MacColl and Debus (6). Within the range of plumage variation, plumages are similar for each sex, although there may be subtle plumage differences within a pair. Essentially adult-like appearance is acquired in Second Basic plumage, although the extent of pale plumage highlights may continue to increase for at least 5 years. The pale plumage features (face, underparts, barring) are correlated with one another (1).

Natal Down

Prepennae down is short and white. Thicker, woollier white preplumulae down emerges apparently by 2 weeks old and is not as bulky as in Peregrine Falcon (Falco peregrinus). The description of natal down by McGilp (12), quoted by Marchant and Higgins (1), appears to be erroneous and possibly based on a misidentified accipitrid raptor nestling.

Juvenile (First Basic) Plumage (HY)

Black Falcon has Juvenile plumage from fledging until late in the first year (around August), when it undergoes a complete molt into second basic plumage (13), with a possible Preformative molt of some body feathers in autumn (May) (see Molts). Juveniles are overall dark sooty or slaty-brown with rufous fringes to the dorsal feathers and a pale tail tip and sometimes pale fringes to the ventral feathers. Most have a white chin, and some have narrow pale barring on the inner webs of the bases of the outer primaries, and narrow pale bars or spots on the tips of the undertail coverts. A few have variably paler cheeks or buff cheeks and forehead, offsetting a thin dark malar stripe, and shadowy narrow pale bars on the inner webs of the tail feathers. Juvenile plumage wears and fades with time, as when approaching the Second Prebasic molt at 1 year of age.

Second Basic Plumage (2Y)

Little precise information. Black Falcon acquires Second Basic (adult-like) plumage late in the first year (13) and retains it through the second year. It apparently undergoes a complete annual molt from Second Basic plumage onwards. Many adults, apparently younger individuals, are overall dark chocolate-brown with variably pale cheeks and a thin dark malar stripe, white chin, narrow pale barring on the bases of the outer primaries, and pale-barred undertail coverts. Dorsal plumage has a "scaled" appearance (narrow pale fringes) when fresh. Some birds have narrow pale barring in the rectrices. Fresh Second Basic plumage wears and fades with increasing time from the Second Prebasic molt. Much individual variation in pale highlights in the plumage may be age-related, increasing with Third Basic plumage (3Y) and later plumages.

Later Basic Plumages (3Y and older)

Pale highlights in the plumage of adults, such as a pale forehead and cheeks, extensive pale throat, speckling on the breast, barring in the primaries and rectrices, speckling on the greater underwing coverts, and the width of the pale bars in the undertail coverts, increase up to at least 5 years of age. A white upper breast is said to develop in individuals older than 10 years (14). In the palest individuals, narrow rectrix barring extends across both vanes and is visible dorsally as shadowy pale bars on the tail. The most extensively speckled and barred individuals appear to be mostly male (6).

Other Plumages

A rare sooty-gray morph was alleged to exist, on the basis of a single report of a pair and juvenile having slate-gray dorsal plumage, the juvenile being sightly darker (1). However, this condition was later considered attributable to the gray wash or bloom sometimes reported in this species, feather wear, and/or perhaps lighting effects (e.g., over-exposed photographic film), superimposed on the range of individual plumage variation, or even to dust-bathing. The paler, more barred adults can have a smoky-gray wash on the mantle and a gray cast to the remiges and rectrices (5, 3). There have been no further reports of a supposed sooty-gray morph.

Molts

The limited information is taken from Marchant and Higgins (1), Charley et al. (9), Debus (15), Whelan et al. (10), Schoeb et al. (11), and Debus (3). Black Falcon exhibits a prebasic molt strategy, with complete annual prebasic molts. No geographic pattern has been identified (e.g., tropics versus temperate zone), and no sex-specific pattern has been suggested other than breeding females starting to molt their primaries a little earlier (in the nestling period) than breeding males (in the fledgling period).

Prebasic molts in Black Falcon follow an inward/outward pattern of flight-feather replacement, starting from the middle primaries. Molt of the primaries starts at P4, proceeding distally and proximally, and is essentially symmetrical. Molt of the secondaries follows a similar pattern, proceeding inward and outward from the middle secondaries at around S5. Molt of the rectrices is outwards/inwards from the central rectrices and r6 in an alternating pattern.

In breeding adults in the eastern temperate and subtropical zones, prebasic molt of the remiges and rectrices lasts from spring (September/October) to early autumn (March/April), with primaries replaced between the fledgling period (or the nestling period in some females) and March. Museum specimens (ages combined) show active primary molt in April, June, and September to December, and body molt in May, with some lacking primary or tail molt in most months of the year. It is possible that May body molt represents a Preformative molt in the first winter, as is known for some other Falco (16), with the June primary molt representing the start of Second Prebasic molt in juveniles fledged in the previous August or September. Primary molt in April represents the finish of post-breeding molt (third prebasic or older).

Prejuvenile (First Prebasic) Molt (HY)

Juvenile plumage emerges and replaces the natal down while the chick is in the nest. White, thicker preplumulae down starts replacing the prepennae down by 2 weeks after hatching. Chicks are downy for the first three weeks, with remiges, rectrices, scapulars, and facial feathers emerging through the down in week 3; from week 4, feathers gradually appear dorsally (including the head) and ventrally (week 5), until in week 6 they are mostly feathered with some down on the crown and underparts. Down persists on the underwings until week 6 or 7 at fledging, when the primaries and tail are still incompletely grown.

Second Prebasic Molt (1 year of age) (2Y)

No definitive information. Acquisition of Second Basic plumage is inferred to start towards the end of the first year, with replacement of remiges and rectrices complete. Molt is stated to occur from August to March (17), here inferred to apply to yearlings. Primary molt in breeding adults takes ~6 months. Body molt in May might represent a Preformative molt in juveniles.

Third and Later Prebasic Molts (3+ years of age) (3Y onwards)

No precise or definitive information, other than that the amount of white in the ventral plumage increases up to at least 5 yr, and possibly up to > 10 yr.

Bare Parts

Information taken from Marchant and Higgins (1), Debus et al. (7), Barnes and Debus (18), Debus and Zuccon (8), Charley et al. (9), Whelan et al. (10), Schoeb et al. (11), and Debus (3). In a few apparently old and mostly male adults with the most "variegated" plumage (pale face, ventral speckling and barring), the cere and feet can be yellow, the orbital ring pale yellow, and the bill can have a yellow-tinged base and gape flanges (6).

Bill and Cere

In downy young, the bill is light gray to light blue-gray, and the cere is pale blue-gray to very pale gray. In fledglings, the bill is blue-gray with a black tip; the cere is brown, sometimes with a slight blue-gray tinge around the nostrils. The cere changes to gray-brown then dark gray by the first autumn (March/April), at which time the juveniles are 7–8 months old. In adults, the bill is black with a pale blue-gray to pale gray base, and the cere is pale blue-gray to pale gray or whitish or (exceptionally) yellow.

Iris and Facial Skin

In downy young, juveniles, and adults, the irides are dark brown. In downy young, the orbital ring is pale blue-gray to very pale gray. In juveniles, the orbital ring is pale blue (fledglings) to pale blue-gray. In adults, the orbital ring is pale blue-gray to pale gray or whitish, apparently lightening with age, or (exceptionally) pale yellow.

Tarsi and Toes

In downy young, the tarsi and toes are pale blue-gray to very pale gray. In juveniles, the tarsi and toes are pale olive-gray (fledglings) to pale blue-gray. In adults, the tarsi and toes are pale blue-gray to pale gray or whitish or (exceptionally) yellow.

Measurements

Linear Measurements

Information taken from Marchant and Higgins (1). Overall body length (bill tip to tail tip) is 45–56 cm, and wingspan 97–115 cm for the sexes combined. Black Falcon is sexually size-dimorphic, with sex differences in means for wing chord, tail, culmen, tarsus, and middle toe lengths. Individuals can be sexed as male if the wing chord is <377 mm and tail <227 mm, and as female if the wing chord is >385 mm and tail >226 mm, although there is potentially some slight overlap between male maxima and female minima for tail length. There are no known age-related differences or geographic variation, and ages were combined in the following samples by Marchant and Higgins (1) because of the difficulty of aging museum specimens on available knowledge at the time.

Wing Length

Male 347–376 mm (mean 362 mm, n = 17).

Female 386–424 mm (mean 405 mm, n = 25).

Bill Length (Exposed Culmen)

Male 22.8–26.9 mm (mean 24.8 mm, n = 17).

Female 25.3–29.9 mm (mean 27.3 mm, n = 24).

Tarsus Length

Male 40.1–46.5 (mean 42.8 mm, n = 9).

Female 43.2–49.8 mm (mean 47.4 mm, n = 9).

Middle Toe Length

Male 41.8–46.6 mm (mean 44.1 mm, n = 11).

Female 40.0–49.6 mm (mean 47.7 mm, n = 11).

Tail Length

Male 200–226 mm (mean 208 mm, n = 17).

Female 227–257 mm (mean 240 mm, n = 26).

Mass

Weights given by Marchant and Higgins (1) might include mis-sexed or emaciated birds (e.g., "female" of 610 g, "male" of 710 g) or, conversely, well-fed captive birds (e.g., female of 1,000 g). Therefore, information is taken from Debus and Olsen (5) and Debus (3). Females are heavier than males. There are no precise data on age-related differences in free-flying birds (adults versus juveniles).

From recent samples of museum specimens and live birds sexed by dissection or by raptor biologists and rehabilitators (5):

Male 481–650 g (mean 582 g, n = 11) (two probable males of 640 and 647 g raise the mean to 592 g)

Female 710–950 g (mean 833 g, n = 18)

Two pre-fledged females weighed 810 and 840 g (5) and a fledgling female 730 g (9), and a male that fledged at normal age and development, but underweight at 470 g, was rehabilitated to flying ability and a release weight of 593 g (19). Another dependent, or nearly independent, flying juvenile male weighed 616 g (8). Additional adult weights of 861 g (female) and 557 g (male) (10, 6) suggest that, for free-flying falcons in good body condition, there is little age-related difference in mass.

Wing Area, Wing Aspect Ratio, Wing Loading

There is no information on these parameters for Black Falcon, other than a calculated wing-loading index based on the formula for wing area for raptors with a falcon wing shape (20). This formula slightly underestimates actual wing loading for falcons by 0.05–0.1 g cm2. Based on the mean wing-chord measurements and weights of free-flying Black Falcons, the calculated average wing area is 1,767 cm2 for males and 2,209 cm2 for females, and wing-loading index for males is 0.34 g/cm2 and for females is 0.38 g/cm2. Black Falcon has longer wings and tail and softer, more flexible flight-feathers than the similar-sized Peregrine Falcon and hence a lower wing loading, conferring greater agility near cover, greater capacity to soar, and the ability to stoop almost to the ground, or even to the ground (21, 22, 17, 5).

Recommended Citation

Debus, S. (2023). Black Falcon (Falco subniger), version 2.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.blafal1.02