Species names in all available languages
|Albanian||Bufi i borës|
|English (United States)||Snowy Owl|
|French||Harfang des neiges|
|French (French Guiana)||Harfang des neiges|
|Romanian||Bufniță de zăpadă|
|Spanish (Spain)||Búho nival|
Bubo scandiacus (Linnaeus, 1758)
- scandiaca / scandiacus
The Key to Scientific Names
Snowy Owl Bubo scandiacus Scientific name definitions
Version: 1.0 — Published March 4, 2020
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Plumages, Molts, and Structure
Snowy Owls have 10 full-length primaries, 18 secondaries (including 3 tertials), and 12 rectrices; owls are diastataxic (see Bostwick and Brady 2002) indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. Wing long and somewhat rounded; p8 longest; outer primaries emarginated and inner vane notched; p10 entirely serrated; p9 serrated on outer tip. Little or no geographic or sex-specific variation in appearance (see Systematics: Geographic Variation) or molt strategies reported.
Following based primarily on detailed plumage descriptions of Ridgway (1914), Dement'ev and Gladkov (1951), Roberts (1955), Oberholser (1974), Cramp (1985), and Potapov and Sale (2012); see Josephson (1980), Pyle (1997a, 1997b), Seidensticker (2011), and Solheim (2012) for age-related criteria. Delayed plumage maturation, sexual color dimorphism, and noticeable individual variation in plumage color unique among world's owls. Definitive Plumage may be attained in 3–4 yr in females but possibly not until 8–10 yr or more in males (entirely or nearly-entirely white plumage); many males do not attain all-white plumage. Extent of black markings also may not be static with age; e.g., adult breeding females can appear lightly marked and white during breeding then dark and heavily marked after molt; but plumage can also appear almost identical in density of markings in marked birds recaptured years apart (D. Zazelenchuk, pers. comm.; Fig. 21).
Remaining questions about plumage in Snowy Owls include: 1) At what age a "definitive" basic plumage is attained for males and females; 2) whether or not all males and females get progressively whiter with age; 3) once a definitive plumage is attained whether or not it is mirrored in each successive plumage; and 4) to what extent bleaching has on black feather markings between fresh fall/winter and worn breeding plumage. Study on known-aged wild Snowy Owls needed to answer most of these questions.
Present primarily Apr–Jun, in the nest. Chicks hatch with complete coat of white protopile (neoptile) down, white or light cream colored. Second coat of gray mesoptile down here considered part of Juvenile Plumage (see below).
Juvenile (First Basic) Plumage
Confusion has existed on whether second down-like feathers should be considered homologous with Juvenile Plumage of other owls and birds, or be considered a second mesoptile down. There appears no subsequent inserted first-cycle molt in Snowy Owls; thus, in the second case the Preformative Molt would be considered absent. Here we follow Oberholser (1984), Cramp (1985), Pyle (1997a) and other recent authors in considering the gray down-like feathers juvenile, coinciding with similar down-like feathers in other owls that vary in quality and structure toward filamentous juvenile feathering of other birds; we consider replacement of these feathers part of Preformative Molt as in other birds. Study needed on homologies of these feather coats in owls and other birds.
Juvenile Plumage present primarily Jun–Aug, during post-natal dispersal on and near natal territory. Upperparts, greater, lesser and median wing coverts, and underparts downy and dark mouse brown, feathers tipped and speckled grayish white, those of scapulars and mantle less filamentous and faintly barred whitish; facial disk and chin white, the feathers tipped brown. Much white neoptile down can still adhere to tips of juvenile rami. Function of sooty-gray down-like feathering presumed to provide thermal advantages; however, it may be compromise between efficient heat absorption of solar radiation and camouflage coloration for protection from predators. After leaving the nest, Snowy Owl chicks can be difficult to find on surrounding tundra, particularly against various shades of black, gray, and white lichens, providing reasonable explanation for sooty-gray coloration [DWH]
Primaries, primary coverts, secondaries, and rectrices of normal pennaceous feather structure, white with dark brown markings as in Definitive Basic female, but feathers narrower and more-tapered at tip. Dark brown markings more numerous and extensive than in later feather generations and more extensive in females than males (Josephson 1980; Cramp 1985; Pyle 1997a, 1997b; Seidensticker et al. 2011).
Male. Tertials and outer primaries white with 3–5 broken bars distally, sometimes with additional spots basally; remainder of secondaries and primaries white basally with some dark spots on outer webs distally; rectrices white basally, the central rectrices (r1) with 2–3 narrow bars or broken spots distally, reduced to spots on r2–r4; r5–r6 usually completely white.
Female. Tertials, secondaries, and primaries with 6–7 full bars distally and with additional broken bars or spots basally; rectrices barred or spotted basally, central rectrices (r1) with 4–5 full bars distally, bars reduced in number and width on r2–r4; r5–r6 white 2–4 broken distal bars.
Between 5-6 weeks old, prior to fledging, juvenile flight feathers developed enough to show sex-specific differences; 140 Snowy Owl aged 38–44 d were correctly sexed based on flight feather markings as confirmed with DNA analysis (Seidensticker et al. 2011, [DWH?]; Fig. 20). On Fetlar Island, Scotland, possible to recognize sex by plumage after 10 weeks (Tulloch 1968). Markings on emerging juvenile flight feathers likely reliable on chicks aged 28–35 d, but best to wait until 5 wk or later (Seidensticker et al. 2011).
"First Basic" or "Basic I" plumage of Humphrey and Parkes (1959) and later authors; see revision by Howell et al. (2003); also considered "Juvenile Plumage" by some authors (see above). Present primarily Oct–Sep. Similar to Definitive Basic female but density and width of dark brown to black markings on body feather and wing coverts greater in female and overlapping or slightly less in male (Josephson 1980; Pyle 1997a, 1997b). Remiges and rectrices uniformly juvenile, without replaced feathers, narrower and more worn than in Definitive Basic Plumage but fresher than in Second Basic Plumage; bars through inner secondaries and outer primaries (including p7) not interrupted (see below). Juvenile gray down may remain in nape through fall (Bent 1938, Keith 1960) but not reliable for ageing thereafter (Josephson 1980).
Male. Plumage white with moderate dark brown to blackish barring and markings; hind-nape feathers and scapulars whitish with sparse broken bars or spots; ventral feathers with 2–5 bars (mean 3.4) of 1–3 mm width (Josephson 1980); undertail coverts white or lightly barred; juvenile remiges and rectrices with less markings, as described under Juvenile Plumage.
Female. Plumage white with heavy dark brown to blackish barring and markings; hind-nape feathers and scapulars with full bars; ventral feathers with 4–6 bars (mean 4.7) of 2–5 mm width (Josephson 1980); undertail coverts barred; juvenile remiges and rectrices with more-extensive markings, as described under Juvenile Plumage.
Second Basic Plumage
Present primarily Oct–Sep in North American populations. Similar to Formative and Definitive Basic Plumage but density and width of bars on body feathers variably intermediate between these two plumages within each sex. Second Basic Plumage best identified by replacement patterns among flight-feathers (Pyle 1997a, 1997b; Solheim 2012). One to two primaries (p7 or p7–p8) often replaced and 4–9 consecutive proximal secondaries (among s11-s18, always including the tertials) replaced, contrastingly broad, glossy, and with fewer and narrower or more-broken bars than adjacent juvenile remiges, resulting in interruption to bar pattern across wing; rectrices vary from completely juvenile and very worn and abraded (rarely) to completely replaced; many show 2-10 feathers replaced, broader, glossier and with fewer markings than retained juvenile rectrices. Some birds do not replace any juvenile primaries but can be recognized by contrasts in feather generations among inner secondaries and very worn retained juvenile outer primaries and rectrices. Most or all Second Basic Males whiter in body plumage (less extensive blackish markings) than most or all Females of all ages; some overlap may occur; sexes best separated by extent of markings on juvenile flight feathers (see Juvenile Plumage).
Third Basic Plumage
Present primarily Oct–Sep. Similar to Definitive Basic Plumage but blackish barring and markings average variably heavier in each sex; Third Basic Plumage best identified by mixed juvenile and basic feathers in wing (Pyle 1997a, 1997b; Solheim 2012). One to 4 primaries (among p6–p10) replaced with 1–2 generations of basic feathers (e.g., p7 Second Basic and p8 and p9 Third Basic); remaining 5–9 primaries (among p1–p6 and p8–p10) juvenile; secondaries with three generations including 4–9 consecutive proximal secondaries Second Basic, 4–12 secondaries (among s1–s6 and s9–s13; perhaps rarely 1–3 tertials as well) Third Basic, and remaining feathers juvenile, usually or always including at least 2–3 juvenile secondaries among s3–s4 and s7–s10; rectrices with one or two generations of basic feathers. Beware replacement patterns among primaries may overlap those of Second Basic or Fourth Basic Plumages, in which case replacement patterns in secondaries and rectrices more helpful for age determination.
Third Basic Males appear to become whiter in body plumage (fewer and smaller markings, but probably never fully white) such that little or no overlap exists with less-extensively marked Females. Density of barring and mottling widely variable and darkest males and whitest females may still be confused; photographs of some marked individuals show only slightly discernible differences in plumage in first few years (Fig.22) while others are noticeably less marked (R. Solheim, N. Smith per. comm.). Study needed based on known-age birds. Age-determination best accomplished by density of markings on retained juvenile flight feathers (see Juvenile Plumage); replaced basic feathers also less-marked in males than in females.
Fourth And Fifth Basic Plumages
Present primarily Oct–Sep. Similar to Definitive Basic Plumages of each sex and in many cases indistinguishable; typically can only be identified by retained juvenile feathers (most often among p3, p10, s4, and/or s9–s10), narrower, more heavily marked (if not bleached), and very abraded, in combination with 2–3 generations of fresher and less-marked basic primaries and secondaries in certain replacement patterns (Pyle 1997a, 1997b; Solheim 2012). Overlap may occur in patterns between Fourth and Fifth basic plumages rendering precise ageing difficult or impossible. Males in these plumages can continue to average whiter than in previous plumages and some may become entirely or almost entirely white (study needed). Females no longer separable from Definitive Basic females based on density and width of blackish feather barring and markings.
Definitive Basic Plumage
Present primarily Oct–Sep.
Male. Body plumage entirely white or white except for narrow, sparse, pale gray, or brown barring on breast, back, wings, head and/or tail; "bib" (distance between lower mandible and uppermost bars on breast, if present) > 100 mm; ventral feathers with 0–2 bars (mean 0.8) of 1–3 mm width (Josephson 1980); other markings, if present, most often small dark brown spots on sides of crown, scapulars, sides of underparts, and/or upperwing coverts (Cramp 1985); primaries and secondaries white or usually with 1–3 broken bars on tertials and outer primaries; remainder of primaries and secondaries sometimes lightly marked; rectrices white or sometimes with broken brown bar on inner 1–3 feathers (r1, less commonly r2–r3). Ear tufts (both sexes) small, numbering about 10, 20–25-mm long (Cramp 1985).
Female. Body plumage and flight feathers white with moderate to extensive barring present on breast, wings, head, and/or tail, most bars crescent-shaped, 9–11 wide at center (Cramp 1985); hind nape, rump, and distal portions of upperwing secondary coverts white with few or no markings; "bib" (distance between lower mandible and uppermost bars on breast) < 80 mm; ventral feathers with 3–6 bars (mean 4.1) of 2–4 mm width (Josephson 1980). Primaries and secondaries white with 2–4 bars, widest and fullest on tertials and outer primaries; remainder of primaries and secondaries with 1–3 broken or full bars; rectrices white with 2–3 bars on inner feathers (r1–r2) and broken bars or spots on r3–r5; outer rectrix (r6) usually unmarked white.
Definitive Basic Plumage distinguished from earlier plumages by lack of juvenile remiges, the feathers being a mixture of 2–5 generations of basic feathers, wider and darker than juvenile feathers, with fewer or no dark markings (Pyle 1997a, 1997b; Solheim 2012).
Although above color differences between sexes in Snowy Owl well known, reasons for this dimorphism have not been discussed in detail. It is suspected that incubating females are difficult to detect as snowmelts off the tundra and increased markings add camouflage during this period. Many worn breeding females appear very white with limited amount of dark barring on feathers and few tail bars; however, as molt initiates and progresses during later stages of nesting, new incoming feathers have much darker and thicker bars; when molt is complete adult females can appear very dark, resembling owls of younger age classes. On wintering grounds, adult males can be more difficult to see in continuous areas covered with fresh snow, whereas females are easier to see. When snow becomes duller and patchier in late winter and early spring, however, females can be more difficult to see and males conspicuous. In Sweden, Lind (1993) concluded females were mobbed less than males during snow free periods due to their darker coloration acting as a better camouflage.
Female Willow Ptarmigan (Lagopus lagopus) can shift feeding areas with plumage as a camouflage tactic (Steen et al. 1992). For example, when white they forage in snow-covered areas, when brown they forage and nest in snow free areas. Whether Snowy Owls use a similar strategy is unknown.
Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Here Snowy Owl is considered to exhibit a Complex Basic Strategy (cf. Howell et al. 2003, Howell 2010), including partial-to-incomplete prebasic molts and a partial preformative molt but no prealternate molts (Stresemann and Stresemann 1966; Oberholser 1974; Cramp 1985; Pyle 1997a, 1997b; Solheim 2012; Figure 2). Study needed on homology of gray natal plumage, considered second mesoptile down by some authors and Juvenile Plumage by others (including here; see Juvenile Plumage, below). If considered a second mesoptile down, the first coat of pennaceous feathers could be considered Juvenile Plumage and the Preformative Molt could be absent, indicating Simple Basic Strategy (Howell et al 2003).
Prebasic molts include complex and bilateral (both distal and proximal) replacement sequence within primaries, as occurs in many owls, with commencement (nodal) position generally occurring more distally in larger genera than in smaller genera (Pyle 1997b, pers. comm.). In Bubo (including both Great Horned and Snowy owl) molt commences at p7 and proceeds bilaterally, but also can commence at p1 and proceed distally before proximal wave reaches p1, resulting in last primary being replaced usually among p2–p4 (Pyle 1997a, 1997b; Solheim 2012). Secondaries in Bubo replaced bidirectionally from s2 (Solheim 2011, 2012), proximally from s5, and distally from the tertials, perhaps with a node at the middle tertial (s17; see Pyle 2013 for more information). Last secondaries replaced in sequence thus among s4 and s7–s10. Rectrices generally replaced distally from r1 on each side of tail, with the outer rectrix (r6) sometimes replaced before adjacent inner rectrices (r4–r5).
Annual prebasic molts variable in extent, often ranging from 0–6 primaries and 4–12 secondaries replaced per molt. Extent likely correlated with lemming cycles, more feathers replaced during years of more-abundant food resources. New replacement sequence commences before all feathers of previous sequence replaced, similar to birds with Staffelmauser molt strategies (Stresemann and Stresemann 1966, Pyle 2006); thus, last juvenile feathers replaced may not occur until Fourth Prebasic Molt or later. Small variations can occur between wings in extent, leading to lack of symmetry in replacement patterns in older birds following multiple prebasic molts (see Pyle 1997b, Solheim 2012). Females appear to average fewer replaced feathers per year than males (Solheim 2012), perhaps related to female's larger size and/or greater energetic demands in reproduction. From no to all rectrices can be replaced per year, all rectrices in older birds during most years (study needed).
Prejuvenile (First Prebasic) Molt
Complete, primarily May–Jun, in nest and during post-natal dispersal near natal territory. By 5–10 d gray down begins to replace white natal down, here considered here part of Prejuvenile Molt (see Juvenile Plumage, below). After young depart nest at ~3 wk, downy juvenile feathers cover body and juvenile primaries and rectrices erupt. Preformative Molt of body feathers (see below) commences prior to full growth of juvenile primaries, which completes during or after fledglings capable of strong flight, at ~50 d.
"First Prebasic" or "Prebasic I" Molt of Humphrey and Parkes (1959) and some later authors; see revision by Howell et al. (2003); here Preformative Molt considered replacement of gray mesoptile down with first coat of pennaceous feathers (see Juvenile Plumage, below). Partial, Jun–Sep (Figure 2), during post-natal dispersal on breeding grounds. Preformative Molt of body feathers may commence as early as 2 wk, when juvenile tarsal and toe feathers start replacing mesoptile down. At about 28–30 d juvenile feathers around face begin to emerge (Watson 1957), followed by head and most body by 38–40 d, and upperwing secondary coverts by 50 d (Cramp 1985). At time of fledging (44-55 days), gray mesoptile down largely replaced except for head ([DWH]). Juvenile remiges, primary coverts, and rectrices retained.
Second Prebasic Molt
Incomplete, primarily May–Oct, occurring on or near breeding grounds (study needed). Averages earlier in year than Definitive Prebasic Molt due to lack of constraints related to breeding in one-year-olds. Includes most or all body feathers and upperwing secondary coverts (study needed on extent of secondary covert replacement during prebasic molts), 0–2 primaries (none, p7 only, or p7–p8; most often p7 only) and corresponding primary coverts, 4–9 proximal secondaries (among s11–s18), and 0 (rarely) to all 12 rectrices; retained rectrices, if present, often among r4–r5 or r4–r6 on each side of tail. See General Molt (above) for more information on sequence.
Third Prebasic Molt
Incomplete, primarily Jul–Oct, occurring on or near breeding grounds. Includes most or all body feathers and upperwing secondary coverts, 1–3 primaries (among p6–p9) and corresponding primary coverts, 4–12 secondaries (among s1–s6 and s9–s13), and usually all 12 rectrices. Replaced primaries and secondaries continue in sequence (see General Molt section) from positions where arrested during Second Prebasic Molt, resulting in retained juvenile primaries and primary coverts among p1–p5 and p9–p10 and retained juvenile secondaries among s1, s3–s4, and s6–s10; 1–3 tertials and perhaps rarely other primaries and secondaries may be replace for second time, commencing a new sequence.
Fourth Prebasic Molt
Similar to Third Prebasic Molt but continues in sequence from where that molt arrested and usually begins new sequence at p7 and among nodal secondaries. Often, perhaps always, 1–10 juvenile remiges may continue to be retained among p2–p4, p10, s3–s4, and/or s7–s10. Study of known-age birds needed as patterns of retained juvenile feathers after Fourth Prebasic replacement in most-advanced individuals likely overlaps those of Fifth Prebasic replacement in least-advanced individuals, rendering age determination impossible based on molt patterns alone.
Definitive Prebasic Molt
Complete, primarily Jul–Oct (Figure 2), on or near breeding grounds. Molt typically begins on breeding grounds with females earlier than males (DWH). Females lose breast feathers with onset of incubation to expose brood patch, perhaps the beginning of molt (DWH). Males begin molt after females, usually in early Jul or later, at time young grow and gain independence (DWH). Two females at Barrow, AK, became flightless while brooding young chicks, apparently due to heavy molt (DWH); after a week they were both able to fly.
Definitive Prebasic Molts includes most or all body feathers and upperwing secondary coverts (study needed), 1–6 primaries and corresponding primary coverts, 4–8 secondaries, and usually all 12 rectrices. Replaced primaries and secondaries continue in sequence from positions where arrested during previous prebasic molt. Following several prebasic molts, remiges can consist of up to 4–5 generations of basic feathers in seemingly random positions. See General Molt (above) for more information on sequence.
Bill And Gape
Bill raptorial and black (mandible and maxilla); mandible strongly downward curved, long, and tapering to point. Nostril large and covered by long, densely packed rictal bristles (DWH, see also Cramp 1985); not known if bristles have tactical/sensory or thermal function. Bristles seem exceptionally long and erected when feedings chicks and are also erected when agitated, such as when encountering predator or being mobbed, during nest defense, or when handled for banding; this all suggests several functions. Gape large and extending back below eye – allowing mouth to open wide for swallowing large prey whole.
Grayish yellow after eyes open about 9–11 d; slowly becomes bluish/yellow at about 22–28 d, and deep yellow as adults ([DWH]). Also, described as bluish-white, cold pale gray, light gray, pale yellow-gray, or greenish yellow at 5–7 d (Cramp 1985). Reported for adults as brilliant golden yellow, at times almost orange (Sutton 1932), golden yellow, lemon yellow (Cramp 1985). Eyelids black on adults (Cramp 1985).
Legs And Feet
At hatching, skin pink, claws/talons white, metatarsal pad pink (see Cramp 1985, [DWH]). As chicks grow, long loosely arranged feathers cover entire leg. Four toes as in all owls, numbered 1–4 from inner to outer; all toes covered with long feathers that extend onto ground when perched; digit 4 opposable and can rotate forward and back as in other owls; claws/talons acutely curved – high arc curvature – long, black, and very sharp. Feathers do not grow on claws or soles of feet.
See Table 1. On average, largest North American owl. Females distinctly larger than males for wing chord and tail length: mean female chord 428, and male 391; mean female tail length 233, and male 217 (Table 1). From Russia, mean wing length for males was 405 mm, range 384-423 (n = 86); and 438 mm, range 428-462 (n = 63), (Dement'ev et al. 1955, in Mikkola 1983), and mean total length for males 585 mm, range 525-640 (n = 9); females 617 mm, range 590-650 (n = 9) (Portenko 1972, in Mikkola 1993). Other data from Europe and Russia found differences between sexes significantly different for: wing, tail, bill, tarsus (see Cramp 1985, and Portenko 1972, 1973, in Cramp 1985).
Previous data from North American based on a small sample of live-trapped or recently killed owls in good physical condition. Mean mass was 1806 g (range 1606-2043; n = 23) for males, and 2279 (1838-2951; n = 21) for females (Kerlinger and Lein 1988a). Females weighed on average 473 g (26.2%) more than males (Kerlinger and Lein 1988a). These weights were significantly heavier than museum weights given by others (Earhart and Johnson 1970, Karalus and Eckert 1974, Snyder and Wiley 1976).
New data from large samples of individuals wintering in Canada and U.S. give the best information. Mean mass males: 1636 g (n = 995), female: 2109 (1189). See Table 1.
For Norway and Russia, see Dementiev and Gladkov (1951), Uspenski and Priklonski (1961), and Portenko (1972), all in Cramp (1985). Also Mikkola (1983).
Fat condition was also assessed in 121 specimens (Kerlinger and Lein 1988b). The authors reported 54 (45%) has moderate to heavy deposits. The remaining 67 (55%) had zero to light fat scores: 24 (20%) had light fat deposits, and 43 (36%) lacked fat. An important point made by the authors was that the 43 owls lacking fat does not mean they were healthy owls that starved. Indeed, several of these owls had sustained other injuries (i.e. broken bones) that led to an inability to hunt – thus starving.
We need more information on live weights of breeding Snowy Owls; e.g., Parmelee et al. (1967) reported one nonbreeding adult male weighed 1,630 g on 27 Jul 1962, and one breeding adult female weighed 2025 g on 28 Aug 1960, when collected in the Canadian Arctic.