Appearance

(see also Systematics: Geographic Variation)

Gyrfalcons have 10 functional primaries, 13 secondaries (including three tertials), and 12 rectrices. Plumage aspect varies extensively across large geographic areas (see Johnson et al. 2007), with "white," "gray/intermediate" and "dark" variants (see below). No geographic variation in molt strategies has been reported.

Molts

Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Gyrfalcon exhibits a Modified Basic Strategy (Howell et al. 2003), including complete prebasic molts and a limited preformative molt in some individuals (Pyle 2005a), but no prealternate molts (Cramp and Simmons 1980; Forsman 1999; Wheeler 2003a, 2003b; Pyle 2008; Fig. 4). The second prebasic molt typically results in definitive plumage aspect, although some juvenal and/or formative feathers can be retained through the second cycle.

Prejuvenile (First Prebasic) Molt

Complete, Jun-Jul, in the nest. Primary 7 emerges at about 11 d and grows in a linear fashion up to about 40–42 d, at about 2 mm/d (Poole 1989). No information on timing or sequence of emergence of contour feathers on different feather tracts, but young are down-covered until about 3 wk and feathered by about 5 wk. Completion of growth of flight feathers does not occur until after fledging.

Preformative Molt

Absent to limited, Nov-Mar, primarily on non-breeding grounds. Can include up to 30% of body feathers but appears to be absent in most individuals (Pyle 2005a, 2008). No wing coverts or flight feathers replaced.

Second Prebasic Molt

Incomplete to complete, Mar-Sep, primarily on breeding grounds (although individuals not breeding during this cycle). Molt continuous without suspensions. Retention of feathers as in Definitive Prebasic Molt (see below) perhaps less common due to lack of energy constraints related to breeding. Dement'ev (1960) suggested that birds undergoing their Second Prebasic Molt may begin body molt in winter and replace flight feathers in spring but this likely based on preformative molt followed by beginning of prebasic molt (cf. Pyle 2005a).

Definitive Prebasic Molt

Incomplete to complete, Apr-Oct, on breeding grounds (Dement'ev 1960, Cramp and Simmons 1980). Primaries and secondaries each replaced both distally and proximally, from centers at p4-p5 and s5 (Miller 1941, Pyle 2005b); e.g., typical sequence of primaries 4–5–6–3–7–2–8–9–1–10. Reports of initiation at p3, p6, or p7 (Waller 1939, Dement'ev and Gladkov 1951) require confirmation. Duration of primary molt 98–127 d (Cramp and Simmons 1980) to approximately 150 d in captive birds (TJC). Tertials molt outward in sequence to meet proximal replacement wave beginning at s5 (Palmer 1988). Rectrices generally replaced distally on each side of tail but r6 usually replaced before medial rectrices; typical sequence 1–2–3–4–6–5 or 1–2–6–3–4–5. Duration of rectrix molt 75–84 d (Cramp and Simmons 1980) to approximately 100 d in captive birds (TJC).

A period of down molt precedes molt of contour feathers and then stops; a second period of down molt occurs at end of body molt. Uncertain whether these 2 episodes of down replacement represent full or partial down molts (TJC). Early replacement of down may be related to brooding young in cold climates; later replacement (during hot weather) may facilitate thermoregulation (Palmer 1988).

Molt of pennaceous feathers begins with primaries and rectrices; secondaries and body feathers follow soon after. In breeding individuals molt can begin during incubation and suspend for chick-feeding after 1-4 medial primaries (among p3-p6), 1-3 medial ss (among s4-s6) and the tertials nave been replaced; molt averages earlier commencement and more feathers replaced before suspension in females than males. No captive birds have been observed to interrupt molt during breeding; on the contrary, if a bird is going to breed, even very late in the season, molt will be delayed; likewise early molt is an indication a bird will not breed in captivity. Scattered wing coverts, body feathers (especially on rump), and occasionally p10 and/or s1 can rarely be retained (Dement'ev and Gladkov 1951, Johnsgard 1990, Sanchez 1993, Pyle 2008; TJC); retention perhaps more common in successful breeders due to energy constraints. Reports that juvenal feathers can be retained through third cycle unlikely and require confirmation.

Plumages

See Dement'ev and Gladkov (1951), Friedmann (1950), Roberts (1955), Cramp and Simmons (1980), Palmer (1988), Cade et al. (1998), Forsman (1999), and Wheeler (2003a, 2003b) for detailed plumage descriptions. Following taken from these and Williams and Matteson (1959), Brown and Amadon (1968), Potapov and Sale (2005), and examination of 68 North American study skins, and 14 live or recently-dead birds by Clum and Cade (1994). Color terminology follows Smithe (1975). All colors observed on Gyrfalcons fall in yellow/yellow-red continuum; all colors tend to be of moderate value (3–7) and poorly saturated (< 4.0). Plumages of all birds contain 2 basic colors: a lighter “background” color and a darker “marking” color resulting in streaks, spots, or bars on feathers. In their quantitative study of plumage coloration in the Gyrfalcon, Potapov and Sale (2005) found no feathers that reflect in the ultra-violet range of the spectrum.

The Gyrfalcon shows extreme variation in plumage color and pattern, exhibiting a range from almost uniform white to uniform brownish-black. Intermediate plumages form a continuous gradation between the two extremes with no distinct breaks or divisions. Although not uniformly agreed upon (see Potapov and Sale 2005), use of terms that describe distinct, identifiable color patterns such as “morph” or “polymorphism” for Gyrfalcons is incorrect (Cade et al. 1998, Flann 2003, Cade 2006) and misleading. For convenience, most birds are lumped into one of three generalized descriptors: white, gray/intermediate, and dark "variants". Each group is described in terms of a range of patterns and colors. White variants have a large proportion of background color to marking color; reverse is true for dark variants; gray/intermediate variants have intermediate amounts of both. The change in proportions results from changes in length and width of shaft-streaks on spotted and streaked feathers (generally on head and ventral body surfaces) and in completeness of barring on barred feathers (generally on mantle, wings, tail, and legs). In complete barring, dark bars are continuous from one lateral edge of feather to the other with bands of light background color between; in incomplete barring, background color on either side of shaft is interrupted in middle by darker marking color. Juveniles and adults, as well as males and females, show differences in proportions of background to marking colors. Adults have smaller or no shaft-streaks on breast, belly, and head, compared to immatures. Ventral surfaces more likely to be spotted and barred rather than streaked; immatures always streaked. Males generally less heavily marked on ventral surfaces, but females often have lighter markings on head.

Natal Down

(May-Jul) uniformly distributed in feather tracts, except sparser in posterior-medial portion of ventral tracts. Color of first down varies in relation to color of future feathers: lightest birds have pure white down and darker birds have a dark wash on down of head and/or back. Second down is lighter in color and denser than 1st and begins to come in at day 8 in captive birds. It emerges from different follicles than 1st down; pinnaceous feathers later erupt from these same follicles, the second down adhering to the feather tips. Young are down-covered until about 3 wk and feathered by about 5 wk.

Juvenal Plumage

(Aug-Jul). Juvenal primaries are thinner and more tapered, and have rounder bars (oval-shaped), when present, than definitive primaries. Juvenal rectricies are narrower and more tapered than definitive rectrices (Pyle 2008).

White variants with background color overall white to a dilution of pale horn color (92). Forehead, crown, and nape finely streaked with Van Dyke brown (121); mantle moderately to heavily marked, having feathers incompletely barred olive brown (28) to sepia (119) with arrowhead tips of same color and broad light margins both terminally and laterally; breast and belly lightly to moderately streaked with Van Dyke brown teardrops; tail clear or lightly to heavily barred olive brown to sepia; primaries with broad irregular subterminal band Van Dyke brown to sepia, variably barred with same color on inner web; secondaries incompletely barred with olive brown to Van Dyke brown; axillaries and underwing coverts with Van Dyke brown shaft-streaks or arrowheads.

Gray/intermediate variants with forehead, crown, nape, and cheeks pale horn color heavily streaked with olive brown to Van Dyke brown; nape sometimes with conspicuous ocelli (eye-like patches) of pale horn; weak facial stripe of Van Dyke brown; mantle pale horn color (occasionally salmon, 6) almost completely barred with olive brown to Van Dyke brown; breast and belly pale horn color moderately streaked with olive brown to Van Dyke brown; tail pale horn to light neutral gray heavily to moderately barred with olive brown or Van Dyke brown, dark marking color sometimes surrounds spots of pale background color (similar to Saker Falcon); primaries pale horn with irregular Van Dyke brown to sepia incomplete barring; secondaries pale horn incompletely barred olive brown to Van Dyke brown; axillaries and underwing coverts barred olive brown to Van Dyke brown with pale horn color margins and spots.

Dark variants with forehead, crown, nape, mantle, and cheeks uniformly Van Dyke brown, sometimes with darker shafts; breast and belly pale horn heavily streaked with olive brown to Van Dyke brown; tail light neutral gray (85) strongly barred with olive brown to Van Dyke brown or uniformly Van Dyke brown; primaries pale horn color with heavy irregular Van Dyke brown to sepia bars on inner webs, and leaving pale horn color spots or speckles on outer webs; secondaries pale horn color heavily barred olive brown to Van Dyke brown; axillaries and underwing coverts heavily barred olive brown to Van Dyke brown leaving minimal pale horn margins and spots.

Second Basic Plumage

(Sep-Aug). Aspect like that of Definitive Basic Plumage (below) but one to a few juvenal wing coverts and/or body feathers (especially on rump) retained, worn, and showing patterns of juvenal feathers. Rectrices and body markings sometimes intermediate in pattern between juvenal and definitive basic patterns. Primaries and secondaries showing even molt clines, (i.e., without "suspension limits"; Pyle 2008).

Definitive Basic Plumage

(Sep-Aug). Definitive basic primaries are broader and more truncate at tip, and have squarer bars, when present, than juvenal primaries. Definitive rectricies are broader and more truncated than juvenal rectrices (Pyle 2008). Individuals in their 3rd cycle or later can be identified by the retention of feathers, as in Second Basic Plumage, but retained feathers proportionally less worn and showing shapes and patterns of definitive feathers. Markings also not intermediate and in some cases (e.g. in extremely white variants) may in certain cases be diagnostic of older individuals but more study is needed.

White variants with background color white; forehead, crown, nape, and cheeks clear to finely streaked with sepia; mantle lightly to heavily marked, having feathers subterminally spotted to completely barred with sepia; breast and belly clear to lightly marked with small sepia teardrops; tail clear or subterminally spotted to strongly barred with sepia; primaries with broad irregular sepia subterminal band, incompletely barred with sepia on inner web; secondaries incompletely barred with sepia; axillaries and underwing coverts clear to lightly marked with sepia shaft-streaks.

Gray/intermediate variants with forehead, crown, nape, and cheeks white to pale horn color lightly to heavily streaked with dark neutral gray (83); nape sometimes with conspicuous ocelli (eye-like patches) of white to pale horn; variably conspicuous malar stripe of dark neutral gray. Mantle background light neutral gray completely barred with dark neutral gray. Breast and belly white to pale horn color (breast occasionally salmon) moderately barred (anteriorly) and spotted (posteriorly) with dark neutral gray. Tail pale horn color to light neutral gray heavily to moderately barred with blackish neutral gray, sometimes surrounding pale spots (similar to Saker Falcon). Primaries white to pale horn color with irregular sepia bars on inner webs, and sepia with irregular white to pale horn color spots on outer webs; secondaries white to light neutral gray incompletely barred with a dilution of sepia; underwing coverts and axillaries white to pale horn color barred with blackish neutral gray.

Dark variants overall Van Dyke brown, except breast and belly background varying from pale neutral gray (86) to pale pinkish buff (121D) heavily streaked (anteriorly) and barred (posteriorly) with Van Dyke brown; tail neutral gray strongly barred with Van Dyke brown, darker color often mottling lighter color, barring faint in darkest individuals; primaries and secondaries pale horn mostly obscured with heavy irregular sepia barring or heavy mottling; underwing coverts and axillaries heavily barred with Van Dyke brown, leaving only small spots or barring of white background.

Bare Parts

Bill

Yellow (white variants) to bluish horn (gray/intermediate and dark variants) with darker tip in first-cycle (sometimes completely dark in juveniles of dark variants) and paler (sometimes with almost translucent tip) in adult male. Adult dark variant bill variable, ranging from blue horn with blackish tip to blackish fading to yellowish horn at base.

Cere and gape greenish bluish gray to greenish (first-cycle) to yellow (adults). Bill and cere colors can be slow to develop, duller in second-cycle than in older birds, and also tend to be brighter in males than in females. Hatchlings with bill paler and cere and gape pinkish to yellowish.

Iris

Dark brown in all ages and variants.

Legs And Feet

Pinkish (hatchlings) to greenish blue-gray (first-cycle) to yellow (adults). Leg color may be intermediate during second cycle and, in adults, averages brighter yellow in males than females. Talons pale horn (white variants) to dark horn or black (dark variants).