Plumages, Molts, and Structure

Siberian Nuthatches have 10 functional primaries (numbered distally from innermost p1 to outermost p10, the p7 longest and the p10 reduced in length), 9 secondaries (numbered proximally from outermost s1 to to innermost s9 and including three tertials, s7-s9 in passerines), and 12 rectrices (numbered distally on each side of the tail, from innermost r1 to outermost r6). No geographic variation in appearance has been described; the following descriptions cover all subspecies and are based on those of Dement'ev and Gladkov (8 Dement'ev, G. P., and N. A. Gladkov (1954). The Birds of the Soviet Union. Volume 5. Moscow. [English translation in 1970 by the Israel Program for Scientific Translations, Jerusalem, and Smithsonian Institution and National Science Foundation, Washington, DC, USA.] Close ), Eck (2 Eck, S. (1984). Katalog der ornithologischen Sammlung Dr. Udo Bährmanns (3. Fortsetzung). Zoologische Abhandlungen, Mu­seum für Tierkunde Dresden 39:71–98. Close , 4 Eck, S. (1996). Die Paläarktischen Vögel — Geospezies und Biospezies. Zoologische Abhandlung 49(Supplement). 103 pp. Close ), Cramp and Perrins (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ), Harrap & Quinn (3 Harrap, S., and D. Quinn (1995). Chickadees, Tits, Nuthatches & Treecreepers. Princeton University Press, Princeton, NJ, USA. Close ), and Red'kin and Konovalova (1 Red’kin, Y., and M. Konovalova (2006). Systematic notes on Asian birds. 63. The eastern Asiatic races of Sitta europaea Linnaeus, 1758. Zoologische Mededelingen Leiden 80(5):241–261. Close ), along with examination of Macaulay Library Images; see these references, Jenni and Winkler (10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ), and Pyle (11 Pyle, P. (2022). Identification Guide to North American Birds, Part I. 2nd edition. Slate Creek Press, Forest Knolls, CA, USA. Close ) for criteria related to ageing and sexing in the formerly conspecific Eurasian Nuthatch (Sitta europaea) and other similar Sitta nuthatches. See Molts for molt and plumage terminology. Variation in plumage by age and sex not well studied in Siberian Nuthatch, but it is likely similar to that of Eurasian Nuthatch, in which sexes differ in appearance, slightly and on average in Juvenile Plumage and more markedly in later plumages, but with differences overlapping within the entire population (see images for Eurasian Nuthatch under Plumages). Definitive appearance is assumed at the Second Basic Plumage.

Natal Down

Present primarily May-July, in the nest cavity. In Eurasian Nuthatchit is sparse, dark gray down is longer on back than on crown (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ).

Juvenile (First Basic) Plumage

Present primarily June-August. Presumably as in Eurasian Nuthatch, in which Formative Plumage is similar to Definitive Basic Plumage in females, but plumage overall somewhat duller, the upperparts browner and sometimes with paler feather centers (especially on forehead) or diffusely pale mottling among distal portions and feather tips (especially upperwing median and greater coverts, which can also be tipped whitish or cinnamon); eye line duller, slightly browner and less well defined (especially female); underparts slightly paler or perhaps creamier and duller. Feathering, especially that of the lower underparts, is loosely textured due to lower barb densities. In addition, bill, gape, and legs are pale yellow or pink in juveniles (see Bare Parts). Juvenile Females likely average duller than Males within each subspecies but differences are not as marked as in later plumages.

Formative Plumage

Often considered "post-juvenile" or "first-adult" plumage under life-cycle terminology. Present primarily September (when fresh) to June (when worn), assuming no First Alternate Plumage (see Molts). Presumably differs from Definitive Basic Plumages in the same way as that of Eurasian Nuthatch: eye line and general body feathering average slightly duller within each sex. Molt limits present between upperpart and upperwing feathers, the back feathers, scapulars, and sometimes some proximal secondary coverts formative, fresher and bluer (male) or grayish blue (female), contrasting with browner-washed and more faded retained outer greater and median coverts, often with diffusely paler distal portions or sometimes whitish tips. Most Eurasian Nuthatchs have a molt limit within the median coverts (10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ). Greater alula, primary coverts, and outer primaries and rectrices narrower, more tapered or pointed at the tips, and become browner and more abraded with wear. Age and sex determination may sometimes be difficult in this species, with overlaps in body feather coloration, particularly between formative males and definitive basic females.

Definitive Basic Plumage

Often considered "post-breeding" or "adult" plumage under life-cycle terminology. Present primarily September (when fresh) to June (when worn), assuming no Definitive Alternate Plumage (see Molts). The following has been adapted from limited general descriptions relative to those of Eurasian Nuthatch and may need specific updating.

Female

Crown, upperparts, and fringing to the upperwing coverts medium-pale bluish gray, the inner webs of the coverts slaty. Central rectrices (r1) dull blue-gray, the remainder blackish with blue-gray tips (broadest on outermost), the outer three pairs (r4-r6) with extensively white outer webs (especially on outermost two pairs) and variably blackish tips. Primary coverts, alula, and remiges dark slaty blackish, the secondaries, inner primaries, and primary coverts fringed pale blue-gray, these fringes widest on the tertials. A distinct brownish-black eye line extends from the lores to the sides of the nape, about the width of the eye and encompassing it. Malar area and auriculars below eye line, along with chin and throat, white to whitish, the lower throat sometimes tinged buff. Remaining underparts variably whitish, creamy, or pale pinkish buff, the flanks without a deeper rufous wash; sides of vent and undertail coverts usually dark with white mottling. Underwing secondary coverts gray; underwing primary coverts black, contrasting with white bases of primaries from below. In worn plumage, upperparts duller and faded to grayish blue; underparts paler.

In both sexes, Definitive Basic Plumage separated from Formative Plumage by having uniformly basic back feathers and wing coverts, the back contrasting only slightly in color but not in feather quality with the wings; median and greater coverts do not show paler distal portions or whitish tips; blackish primary coverts show broader pale grayish-blue to bluish edging, not contrasting in feather quality with greater coverts; basic outer primaries and rectrices broader, blacker (less brown), and relatively fresher. Body feathering also varies in brightness by age within each sex, the eye line averaging more slaty or blackish and the upperparts bluer (less gray).

Male

Similar to definitive basic female but eye line bolder and blacker, upperparts brighter blue, and underparts bright white with contrastingly bright and rich rufous feathering to the flanks; the presence or absence of dark rufous in the flanks is likely the best criterion for sexing, as in Eurasian Nuthatch (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ). Basic remiges and rectrices average blacker and glossier in definitive basic males than females.

General

Molt and plumage terminology follows Humphrey and Parkes (12 Humphrey, P. S., and K. C. Parkes (1959). An approach to the study of molts and plumages. Auk 76(1):1–31. Close ) as modified by Howell et al. (13 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. Condor 105:635–653. Close ). Under this nomenclature, terminology is based on evolution of molts along ancestral lineages of birds from ecdysis (molts) of reptiles, rather than on molts relative to breeding season, location, or time of the year, the latter generally referred to as “life-cycle” molt terminology (10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ). In north-temperate latitudes and among passerines, the Humphrey-Parkes (H-P) and life-cycle nomenclatures correspond to some extent but terms are not synonyms due to the differing bases of definition. Prebasic molts often correspond to “post-breeding“ or “post-nuptial“ molts, preformative molts often correspond to “post-juvenile“ molts, and prealternate molts often correspond with “pre-breeding“ molts of life-cycle terminology. The terms prejuvenile molt and juvenile plumage are preserved under H-P terminology (considered synonyms of first prebasic molt and first basic plumage, respectively) and the former terms do correspond with those in life-cycle terminology.

Molting strategies are unstudied in Siberian Nuthatch but here assumed to parallel those of Eurasian Nuthatch (Sitta europaea), as described and referenced below. These species appear to exhibit the Complex Basic Strategy ( 13 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. Condor 105:635–653. Close , 14 Howell, S. N. G. (2010). Molt in North American Birds. Houghton Mifflin Harcourt, Boston, MA, USA. Close ), including complete prebasic molts and a partial preformative molt but no prealternate molts (8 Dement'ev, G. P., and N. A. Gladkov (1954). The Birds of the Soviet Union. Volume 5. Moscow. [English translation in 1970 by the Israel Program for Scientific Translations, Jerusalem, and Smithsonian Institution and National Science Foundation, Washington, DC, USA.] Close ,9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close , 3 Harrap, S., and D. Quinn (1995). Chickadees, Tits, Nuthatches & Treecreepers. Princeton University Press, Princeton, NJ, USA. Close , 15 Shirihai, H., and L. Svensson (2018). Handbook of Western Palearctic Birds. Volume 1. Passerines: Larks to Warblers. Bloomsbury, London, UK. Close ,10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ). Prealternate Molts appear not to occur ( 15 Shirihai, H., and L. Svensson (2018). Handbook of Western Palearctic Birds. Volume 1. Passerines: Larks to Warblers. Bloomsbury, London, UK. Close ,10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ), including among examination of 200+ specimens of (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ) and extensive review of Macaulay Library images for Eurasian Nuthatch; however, very limited molts of a few feathers may possibly occur, as found in other Sitta nuthatches (16 Banks, R. C. (1978). Prealternate molt in nuthatches. Auk 95:179–181. Close , 11 Pyle, P. (2022). Identification Guide to North American Birds, Part I. 2nd edition. Slate Creek Press, Forest Knolls, CA, USA. Close ).

Prejuvenile (First Prebasic) Molt

Complete, primarily May-July, in the nest. There is little information on timing or sequence of Prejuvenile molt in this species or Eurasian Nuthatch.

Preformative Molt

Partial, primarily July-September; in Eurasian Nuthatch reported to begin at 2 mo of age (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ) and includes body feathers, most to all upperwing lesser coverts, usually at least one but not all median coverts (83% replaced the inner 4-6 coverts), and rarely 1-2 inner greater coverts but no other wing or tail feathers (10 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ).

Definitive Prebasic Molt

Complete, primarily June-September; in Eurasian Nuthatch it can begin as early as late May in some individuals, primarily those that have failed or skipped breeding (17 Matthysen, E. (1986). Postnuptial moult in a Belgian population of Nuthatches Sitta europaea. Bird Study 33:206–213. Close ,9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close ), but molt is reported to be more compressed in Siberian Nuthatch (3 Harrap, S., and D. Quinn (1995). Chickadees, Tits, Nuthatches & Treecreepers. Princeton University Press, Princeton, NJ, USA. Close ). Primaries and corresponding primary coverts are replaced distally (p1 to p10), secondaries are replaced bilaterally from the middle tertial (s8) and proximally from s1, with the last feather replaced usually s6, and rectrices are often replaced distally on each side of the tail, although variation in rectrix-replacement sequence may occur. In Eurasian Nuthatches of Belgium, the total duration of molt was estimated as about 88 d; molt of secondaries was completed a few days after the end of primary molt; inner primaries molted faster than outer primaries; and no differences in molt were found between sexes or age classes (17 Matthysen, E. (1986). Postnuptial moult in a Belgian population of Nuthatches Sitta europaea. Bird Study 33:206–213. Close ).

The following is based on descriptions of bare parts (9 Cramp, S., and C. M. Perrins, Editors (1993). The Birds of the Western Palearctic. Volume 7: Flycatchers to Shrikes. Oxford University Press, Oxford, UK. Close , 3 Harrap, S., and D. Quinn (1995). Chickadees, Tits, Nuthatches & Treecreepers. Princeton University Press, Princeton, NJ, USA. Close ) along with examination of Macaulay Library images for Eurasian Nuthatch (Sitta europaea). Study is needed on how bare-part coloration may differ in Siberian Nuthatch.

Bill And Gape

The bill is stout with a chisel-shaped tip. In adults it is dark gray to black, darkest on culmen and tip, with the base of the mandible bluish white or horn-gray. In nestlings and juveniles the gape is swollen and cream to bright yellow and the bill can be mostly yellow with a dusky tip. During the first year it quickly becomes adult-like in color.

Iris

At all ages the iris is brown to blackish brown.

Tarsi and Toes

Legs and feet are variable in color. In adults they are usually dark gray to pale brown but can be tinged yellow or even fully yellowish. In nestlings they are pink, becoming yellowish as Juveniles and darker and grayer during the first year

Linear Measurements

Total Length Male: 147.97 mm ± 0.52 SE (range = 140.9–153.0 mm, n = 3). Female: 150.67 mm ± 0.48 SE (range = 144.0–155.0 mm, n = 3).

Wing Span Male: 256.67 mm ± 0.13 SE (range = 255.0–259.0 mm, n = 3). Female: 251.67 mm ± 0.18 SE (range not give; n = 3).

Wing Length Male: 86.42 mm ± 0.15 SE (range = 83.8–90.0 mm, n = 13). Female: 83.86 mm ± 0.22 SE (range = 81.3–87.7 mm, n = 7).

Tail Length Male: 49.85 mm ± 0.20 SE (range 47.9–52.5 mm, n = 14). Female: 46.71 mm ± 0.22 SE (range 44.8–49.0 mm, n = 7).

Tarsus Length Male: 16.99 mm ± 0.16 SE (range = 15.6–17.8, n = 14). Female: 16.91 mm ± 0.11 SE (range = 16.4–17.5, n = 7).

Bill Length Male: 18.61 mm ± 0.20 SE (range = 17.0–19.6 mm, n = 13). Female: 19.05 mm ± 0.13 SE (range = 18.6–20.1 mm, n = 6).

Bill Depth Male: 4.10 mm ± 0.11 SE (range = 3.8–4.3 mm; n = 13). Female: 4.10 mm ± 0.13 SD (range = 3.7–4.4 mm; n = 6).

Bill Width Male: 5.36 mm ± 0.09 SE (range = 5.1–5.6 mm; n = 14). Female: 5.39 mm ± 0.10 SE (range = 5.2–5.7 mm ; n = 7).

Hind Toe Length Male: 10.65 mm ± 0.08 SE (range = 10.3–11.2 mm; n = 14). Female: 10.38 mm ± 0.14 SE (range = 9.6–10.8 mm; n = 7).

Hind Toe Claw Length Male: 10.49 mm ± 0.20 SE (range = 9.7–12.0 mm, n = 14). Female: 10.59 mm ± 0.17 SE (range = 9.8–11.2 mm, n = 7).

Data from (1 Red’kin, Y., and M. Konovalova (2006). Systematic notes on Asian birds. 63. The eastern Asiatic races of Sitta europaea Linnaeus, 1758. Zoologische Mededelingen Leiden 80(5):241–261. Close ).

Mass

Male = 21.21 g ± 0.47 SE (17.2–23.5 g, n = 7). Two in late May = 20.4 g and 22.2 g.

Female = 19.7–21.0 g, n = 2

Recently fledged juveniles (< 1 month out of nest): 21.8 g ± 1.8 (18–23.5 g; n = 9)

Data from (1 Red’kin, Y., and M. Konovalova (2006). Systematic notes on Asian birds. 63. The eastern Asiatic races of Sitta europaea Linnaeus, 1758. Zoologische Mededelingen Leiden 80(5):241–261. Close , 18 Tomkovich, P. S. (2008). Birds of the upper Anadyr River (Chukotka Autonomous Area). Collection of Works of the Zoological Museum of Moscow State University, Number 49. Close ).