Plumages, Molts, and Structure

The Spoon-billed Sandpiper has 10 full-length primaries (numbered distally, from innermost p1 to outermost p10), 14–15 secondaries (numbered proximally, from innermost s1 to outermost s9 or s10, and including 4–5 tertials, numbered distally, t1 to t5), and 12 rectrices (numbered distally, from innermost r1 to outermost r6, on each side of the tail). Calidridine sandpipers are diastataxic (5 Bostwick, K. S., and M. J. Brady (2002). Phylogenetic analysis of wing feather taxis in birds: Macroevolutionary patterns of genetic drift? Auk 119(4):943–954. Close ), indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. Little or no geographic variation in appearance has been described (see Systematics). The following is based primarily on descriptions in Dement'ev and Gladkov (6 Dement’ev, G. P., and N. A. Gladkov (1951). Birds of the Soviet Union. Volume 3. Gosudarstvennoe Izdatel’stvo "Sovetskaya Nauka," Moscow, Russia. Close ), Hayman et al. (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ), Tomkovich (8 Tomkovich, P.S. (1992). Spoon-billed Sandpiper in north-eastern Siberia. Dutch Birding. 14(2): 37–41. Close ), and Piersma et al. (9 Piersma, T., J. van Gils, and P. Wiersma (1996). Family Scolopacidae (Sandpipers, Snipes and Phalaropes). In Handbook of the Birds of the World. Volume 3: Hoatzin to Auks (J. del Hoyo, A. Elliott, and J. Sargatal, Editors). Lynx Edicions, Barcelona, Spain. pp. 444–533. Close ), along with review of Macaulay Library images; see Pyle (10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ) for specific criteria to determine age and sex in closely related Calidris sandpipers. See Molts for molt and plumage terminology. Sexes similar in Juvenile, Formative, and Basic Plumages and possibly may show average differences in Alternate Plumages. Definitive appearances are assumed following the Second Prebasic and Second Prealternate Molts.

Natal Down

Present June–July, on the natal site. Natal down described as "light rusty yellow-brown" with "underparts white with black speckles and white markings" (6 Dement’ev, G. P., and N. A. Gladkov (1951). Birds of the Soviet Union. Volume 3. Gosudarstvennoe Izdatel’stvo "Sovetskaya Nauka," Moscow, Russia. Close ). The crown consists of a dark cap with prominent white spots, surrounded by yellowish buff. The remainder of the upperparts are similar to the crown, dark blackish with a sprinkling of stellate spots. The spatulate shape to the bill is present at hatching.

Juvenile (First Basic) Plumage

Present primarily August–October, including through autumn migration. Similar to Definitive Basic Plumage but feathering more even in pattern and wear and with scaled appearance due to pale feather fringing to upperpart feathers. Forecrown white; center crown dark brown to cinnamon-buff, streaked dusky and buff to whitish; nape whitish with fine black streaks, forming hind collar. Upperpart feathers and upperwing coverts blackish basally, with distinct white to rufous fringing, that of the central scapulars more rufous to cinnamon, those of the back and wing coverts whiter. Rectrices narrower and more tapered at the tips than formative and basic feathers. Sides of head and broad supercilium white, the lores and upper auriculars grayish with fine black streaking forming widening eyeline. Underparts white with variable pale buff wash across breast; sides of breast with brownish-buff patch and indistinct darker streaking.

Formative Plumage

Present primarily November–April and perhaps occasionally through September if the First Prealternate Molt is absent (see Molts) . Similar to Definitive Basic Plumage but separated by retained juvenile feathers in the wing and tail. The scapulars and a few upperwing lesser and median coverts are replaced, gray with white fringing, contrasting with more worn and browner retained juvenile median and greater coverts, duller, browner and more worn, the white fringing worn down. Retained juvenile rectrices are narrow, tapered at the tips, and worn, the central rectrices (r1s, perhaps occasionally r2s) sometimes replaced and contrastingly fresh. Primaries and secondaries uniformly juvenile, relatively narrow, brownish, worn, and not showing molt clines indicating replacement (see below).

First Alternate Plumage

Present primarily May–September, although may not be present in occasional individuals in which First Prealternate Molt may be absent (see Molts). Plumage variable, from resembling Formative Plumage but worn to resembling Definitive Alternate Plumage but typically or always with less reddish coloration; extent of reddish may vary on average by sex as in Definitive Alternate Plumage but duller and less extensively reddish (see images below). Molt limits and contrasts between retained juvenile and replaced formative wing and tail feathers as in Formative Plumage but juvenile feathers become browner and quite frayed, especially including the outer primaries if retained. Some birds in worn Formative or First Alternate Plumage may remain to over-summer on winter grounds; these typically do not develop much or any rufous coloration to the head and breast (either the First Prealternate Molt is skipped or first alternate feathers are replaced but resemble formative feathers).

Definitive Basic Plumage

Present primarily November–March. Forecrown white; center crown gray with diffuse dusky and white streaking; nape paler forming indistinct hind collar; back, scapulars, and central upperwing lesser and median coverts, and tertials gray, the feathers fringed white, forming scaly appearance, and with dusky shaft streaks. Rump and uppertail coverts blackish centrally and white laterally. Rectrices gray with white fringing. Marginal upperwing coverts (along leading edge of wing) and primary coverts blackish, the latter tipped white when fresh; greater coverts dark gray with moderately broad white tips forming wing stripe in flight; remiges dusky gray with narrow white tips when fresh, the tips and outer webs of the outer primaries becoming blacker distally. Lores, suppercilium, and sides of head primarily white with dark brown and streaked auricular patch. Underparts white except for small gray patch with dusky streaks on sides of neck and breast. Underwing coverts white except for marginal lesser and outer primary coverts washed darker gray.

Definitive Basic Plumage separated from Formative Plumage by basic upperwing coverts uniform in wear and quality, gray with white fringes; basic tertials uniform in wear within tract and with other secondaries; basic primaries duskier, fresher, and broader at tips, molt clines occur from more worn inner to fresher outer primary and among secondaries in sequence, with the outer secondary appearing fresher than the inner primary; rectrices uniformly broader, less tapered at the tips, and fresher than retained juvenile rectrices of Formative Plumage.

Definitive Alternate Plumage

Present primarily April–September. Similar to Definitive Basic Plumage but scattered to most upperpart feathers and scattered upperwing coverts, and often 1-3 tertials replaced, the alternate feathers with black centers and broad rufous and white fringing that wears off, producing a darker appearance in summer and early fall. The upperpart pattern provides excellent camouflage in their tundra breeding grounds (8 Tomkovich, P.S. (1992). Spoon-billed Sandpiper in north-eastern Siberia. Dutch Birding. 14(2): 37–41. Close ). Head, throat, and upper breast variably colored reddish, from dull rust or brownish red to bright orange-rufous. Examination of Macaulay Library images indicates that plumage coloration may be bimodal according to sex, varying from dull brownish red with white and brown mottling, especially to the crown, and a paler throat and breast (probably females), to bright orange-rufous throughout the head, throat and breast (probably males) but confirmation of this sex-specific plumage is needed. Males may also replace more feathers on average than females (see Molts). If present these could match sex-specific characteristics in other similar Calidris sandpipers such as Red-necked Stint (Calidris ruficollis) (probably) and Sanderling (Calidris alba)(10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ).

Definitive Alternate Plumage is separated from First Alternate by fuller breeding appearance and lack of molt limits and other features described under Definitive Basic Plumage. The reddish coloration to the head and breast also averages brighter in Definitive than in First Alternate Plumage, sex for sex.

General

Molt and plumage terminology follows Humphrey and Parkes (11 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. (12 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 (13 Pyle, P., S. N. G. Howell, D. I. Rogers, and C. Corben (2024). Moult terminology: Envisioning an evolutionary approach. Journal of Avian Biology 2024:e03169. Close ), rather than on molts relative to current breeding seasons, locations, or time of the year, the latter generally referred to as “life-cycle” molt terminology (14 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. 2nd edition. Bloomsbury, London, UK. Close ; see also 15 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ). In north-temperate latitudes, 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 (15 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ).

Spoon-billed Sandpiper exhibits a Complex Alternate Strategy (cf. 12 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 ), including a complete Definitive Prebasic Molt, a partial Preformative Molt, an absent-to-limited First Prealternate Molt, and a limited-to-partial Definitive Prealternate Molt. As such, the Spoon-billed Sandpiper exhibits what has been referred to as a "Northern Hemisphere Molting Strategy" (pp. 500–505 in 10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ), including less protracted molts (usually completed by winter) and Preformative Molts that are less extensive and do not include outer primaries in eccentric sequences. It may be possible that a second inserted Definitive Presupplemental Molt occurs during spring (see below).

Prejuvenile (First Prebasic) Molt

Complete, late June–July, at natal site. No information on this molt in Spoon-billed Sandpiper.

Preformative Molt

Partial, primarily September to December, likely primarily at stopover sites in China (see Definitive Basic Plumage, below). Examination of Macaulay Library images (see those under Formative Plumage) indicates that it includes most to all body feathers, some scattered proximal upperwing lesser and median coverts, and occasionally 1–2 tertials (t2 only or t1–t2), but no greater or primary coverts, primaries, or secondaries besides the tertials. All rectrices appear to be retained in most birds although replacement of 1–2 central feathers (r1s) might be expected in some birds.

First Prealternate Molt

Absent to limited, primarily April–June. Molt can likely commence on winter grounds but can complete at northbound stopover sites (see Definitive Alternate Plumage, below). Examination of Macaulay Library images (see those under First Alternate Plumage) indicates that it may be absent in some birds but can include scattered back feathers and wing coverts in most birds. Head and breast feathers may also get replaced with either whitish or reddish feathers. Individuals over-summering on winter grounds likely replace fewer feathers on average, as in other Calidris sandpipers (10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ; see middle image under First Alternate Plumage).

Second and Definitive Prebasic Molts

Complete, primarily August–October. Primaries are replaced distally (p1 to p10), secondaries replaced bilaterally from the 2nd or 3rd tertial and proximally from s1 and perhaps s5, and rectrices are generally replaced distally (r1 to r6) on each side of tail, with some variation occurring. The Second Prebasic Molt likely averages earlier in timing, especially for over-summering birds, as in other Calidris sandpipers that may not breed as yearlings (10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ). Molt of primaries possibly may be suspended in some birds, either for migration from breeding to molting grounds (often in Calidris after 2–4 inner primaries replaced) or migration from molting to wintering grounds (e.g., see central image below). Generally, tertials begin replacement when primary molt has reached p2–p3, rectrices at p4–p5, and outer secondaries when primary molt has reached p5–p7 (sometimes secondaries can be replaced in large blocks or nearly synchronously).

The prebasic molts take place primarily at key stopover sites along its migratory route, particularly in intertidal areas of the Yellow Sea. These sites are essential not only for replenishing the energy reserves required for migration but also for the crucial process of feather replacement following the breeding season. Of particular significance are the Jiangsu coast of China and possibly areas in the Democratic People’s Republic of Korea, both of which play a vital role in this stage of the sandpiper's annual cycle (16 Chang, Q., T. L. T. Ki, G. Q. A. Anderson, K. Brides, N. A. Clark, J. Ding, K. K. S. Leung, J. Li, D. S. Melville, J. Phillips, E. Weston, Z. Yang, and R. E. Green. (2021). Numbers of Spoon-billed Sandpipers in Jiangsu Province, China, during the post-breeding moult in relation to recent changes in the intertidal zone. Wader Study 128:125–136. Close ). Recent research indicates that the Spoon-billed Sandpiper spends extended periods in the Jiangsu coast of China during their molt, staying for an average of 66 days. Molting typically begins in early August and continues through late October. This region supports the bulk of the global population during molt, providing the rich food resources needed for feather regrowth and energy replenishment. The prolonged stopover period at the Jiangsu coast (especially Tiaozini wetland) underscores its irreplaceable role in the species' life cycle and conservation (17 Yang, Z., B. J. Lagassé, H. Xiao, M. V. Jackson, C.-Y. Chiang, D. S. Melville, K. S. K. Leung, J. Li, L. Zhang, H.-B. Peng, X. Gan, W.-L. Liu, Z. Ma, and C.-Y. Choi (2020). The southern Jiangsu coast is a critical moulting site for Spoon-billed Sandpiper Calidris pygmaea and Nordmann’s Greenshank Tringa guttifer. Bird Conservation International 30(4):649-660 Close ).

Definitive Prealternate Molt

Partial, primarily March–May on winter grounds but can complete at stopover sites in some individuals, where it may also be possible that an extra Definitive Presupplemental Molt occurs. The Definitive Prealternate Molt includes some to most upperpart, head, and breast feathers, scattered proximal upperwing secondary coverts, often 1–3 tertials, and sometimes 1–4 central rectrices, a pattern typical of this molt in Calidris (10 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ). It may be possible that a second inserted Definitive Presupplemental Molt occurs during spring, with alternate feathers replaced on winter grounds, basic-like in coloration, being replaced again at stopover sites (including with reddish feathers to the head and breast) during northbound migration, as has been documented in other Calidris species (18 Battley, P. F., D. I. Rogers, and C. J. Hassell (2006). Prebreeding moult, plumage and evidence for a presupplemental moult in the Great Knot Calidris tenuirostris. Ibis 148(1):27–38. Close , 19 Pyle, P. (2019). Sunset Sanderlings: Digital photography leads to novel insights about the presupplemental molt of the Sanderling. Birding 52:30–41. Close and cf. right-hand image below); study is needed.

The following is based on descriptions in the literature (6 Dement’ev, G. P., and N. A. Gladkov (1951). Birds of the Soviet Union. Volume 3. Gosudarstvennoe Izdatel’stvo "Sovetskaya Nauka," Moscow, Russia. Close , 7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close , 9 Piersma, T., J. van Gils, and P. Wiersma (1996). Family Scolopacidae (Sandpipers, Snipes and Phalaropes). In Handbook of the Birds of the World. Volume 3: Hoatzin to Auks (J. del Hoyo, A. Elliott, and J. Sargatal, Editors). Lynx Edicions, Barcelona, Spain. pp. 444–533. Close ), along with examination of Macaulay Library images.

Bill and Gape

The most distinctive feature of the Spoon-billed Sandpiper is its spatulate or "shovel-shaped" bill, adapted for tactile foraging in mud and shallow water, enabling it to efficiently detect prey like small invertebrates (20 Kelly, C., C. Zöckler, B. Scampion, and E. E. Syroechkovskiy (2017). Hammer, filter or microphone: How does the spoon-billed sandpiper calidris pygmaea use its bill to feed?. Wader Study 124:99–111. Close ). The spatulate tip is present in hatchlings (cf. ) and is fully developed in juveniles. The bill is black at all ages and year-round. it may vary to being paler or pinkish at the base and with a swollen gape in nestlings (study needed).

Iris and Facial Skin

At all ages the eyes are dark brown but appear blackish in the field.

Tarsi and Toes

In juveniles and adults, the legs and feet are dark gray or black. They may vary to being paler or pinkish in nestlings (study needed). The feet can appear yellowish in some first-year birds (cf. , possibly simply due to staining from vegetation but possibly true coloration. Webbing between the toes is lacking or nearly so (cf. ).

Linear Measurements

The Spoon-billed Sandpiper typically measures between 14–16 cm in total length (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ), with a wingspan ranging from 32–38 cm. Measurements given in mm as means with ranges in parentheses.

Bill Length. Adult male 21.2 (19.6–22.3), n = 14; juvenile male 22.6 (22.0–23.0), n = 3; adult female 22.8 (21.7–24.0), n = 9; juvenile female 20.1, n = 1 (21 Portenko, L. A. (1957). Studien an einigen selten Limicolen aus dem nördlichen und östlichen Sibirien. I. Die Löffelschnepfe - Eurynorhynchus pygmeus (L.). Journal of Ornithology. 98(4):454–466. Close ). Male 21.12 ± 0.70 SD (range 19.9–22.7), n = 65; female 22.34 ± 0.83 SD (range 20.6–24.3), n = 61 (8 Tomkovich, P.S. (1992). Spoon-billed Sandpiper in north-eastern Siberia. Dutch Birding. 14(2): 37–41. Close ). Adult 19–24 mm (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ).

Spoon Width. Adult male 10.7 (10.1–11.2), n = 13; juvenile male 11.3 (10.9–12.0), n = 3; adult female 11.1 (10.7–11.6), n = 8; juvenile female 9.5, n = 1 (21 Portenko, L. A. (1957). Studien an einigen selten Limicolen aus dem nördlichen und östlichen Sibirien. I. Die Löffelschnepfe - Eurynorhynchus pygmeus (L.). Journal of Ornithology. 98(4):454–466. Close ). Adult 10–12 mm (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ).

Wing Length. Adult male 98.9 (95.3–102.4), n = 14; juvenile male 100.4 (98.8–102.2), n = 3; adult female 99.9 (95.5–103.5), n = 9; juvenile female 100.9, n = 1 (21 Portenko, L. A. (1957). Studien an einigen selten Limicolen aus dem nördlichen und östlichen Sibirien. I. Die Löffelschnepfe - Eurynorhynchus pygmeus (L.). Journal of Ornithology. 98(4):454–466. Close ). Male 104.4 mm ± 1.5 SD (range 101–107), n = 65; females 106.9 mm ± 1.8 SD (range 103–111), n = 61 (8 Tomkovich, P.S. (1992). Spoon-billed Sandpiper in north-eastern Siberia. Dutch Birding. 14(2): 37–41. Close ). Adult 98–106 mm (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ).

Tarsus. Adult male 20.9 (20.1–21.8), n = 14; juvenile male 21.1 (20.3–21.7), n = 3; adult female 21.6 (20.4–22.4), n = 8; juvenile female 20.5, n = 1 (21 Portenko, L. A. (1957). Studien an einigen selten Limicolen aus dem nördlichen und östlichen Sibirien. I. Die Löffelschnepfe - Eurynorhynchus pygmeus (L.). Journal of Ornithology. 98(4):454–466. Close ). Adult 19–22 mm (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ).

Tail. 37–39 mm (7 Hayman, P., J. Marchant, and T. Prater (1986). Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin Company, Boston, Massachusetts, USA. Close ).

Mass

Males generally weigh around 29.5 grams, while females can weigh up to 34 grams (21 Portenko, L. A. (1957). Studien an einigen selten Limicolen aus dem nördlichen und östlichen Sibirien. I. Die Löffelschnepfe - Eurynorhynchus pygmeus (L.). Journal of Ornithology. 98(4):454–466. Close , 22 Day, R. H., E. P. Knudtson, D. W. Woolington, and R. P. Schulmeister (1979). Caprimulgus indicus, Eurynorhynchus pygmeus, Otus scops and Limicola falcinellus in the Aleutian Islands, Alaska. The Auk 96(1):189-190. Close ).