Plumages, Molts, and Structure

Sharp-tailed 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 (18 Bostwick, K. S., and M. J. Brady (2002). Phylogenetic analysis of wing feather taxis in birds: Macroevolutionary patterns of genetic drift? Auk 119:943–954. 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 (19 Dement'ev, G. P., R. N. Meklenburtsev, A. M. Sudilovskaya, and E. P. Spangenberg (1968). Birds of the Soviet Union. Volume 2. Israel Program for Scientific Translations, Jerusalem, Israel. Close ), Cramp and Simmons (20 Cramp, S., and K. E. L. Simmons (1983). The Birds of the Western Palearctic, Volume 3: Waders to Gulls. Oxford University Press, Oxford, UK. Close ), Hayman et al. (21 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 ), and Higgins and Davies (22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ), and (23 Wells, D. R. (1999). The Birds of the Thai-Malay Peninsula. Volume 1. Non-passerines. Academic Press, London, UK. Close ), along with review of Macaulay Library images; see these sources, Prater et al. (24 Prater, A. J., J. H. Marchant, and J. Vuirinen (1977). Guide to the Identification and Aging of Holarctic Waders. British Trust for Ornithology, Tring, United Kingdom. Close ), and Pyle (25 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 information on criteria to determine age and sex. See Molts for molt and plumage terminology. Sexes similar in all plumages; definitive appearances usually assumed at second basic and alternate plumages.

Natal Down

Present in June–July, on the natal site. Undescribed in Sharp-tailed Sandpiper. In most Calidris sandpipers hatchlings are covered with thick down; upperparts mottled buff, blackish, and reddish and lower underparts white. Upperpart coloration adapted to provide cryptic coloration against tundra nesting habitats.

Juvenile (First Basic) Plumage

Present primarily August–October. Differs rather substantially from later plumages, unusually so for Calidris sandpipers. Similar in overall head and upperpart pattern to Definitive Basic Plumage but crown reddish with black streaks, creating well-defined cap bordered by prominent white lores and supercilium. Upperpart feathering, upperwing coverts, and tertials dark with even rufous and buff fringes creating an evenly scaled appearance; feathers tipped or fringed narrowly with white when fresh. Rump, central uppertail coverts, and central rectrices dark brown with rufous feather fringes; lateral rectrices brownish with narrow white fringes (cf. ML478304501). Sides of head often tinged buff ; lores and auricualrs brown or slightly tinged rufous forming eyeline. Juvenile primary coverts and remiges (other than tertials) similar to basic feathers but evenly fringed whitish. Sides of neck and breast bright cinnamon buff, unstreaked centrally but with diffuse black streaks or mottling on the sides of the breast. Remaining underparts white. Juvenile flight feathers narrower and more tapered or rounded at the tips than formative and basic feathers, fresh at a time of year when older birds show abraded flight feathers.

Formative Plumage

Present primarily December–March, perhaps occasionally through September. Plumage variable, especially during transitional stages, showing a mix of cinnamon-buff characters of juvenile plumage and grayer appearance of Definitive Basic Plumage. Following completion of molt in January–March, Formative Plumage very similar to Definitive Basic Plumage and often can be identified only by retained juvenile flight feathers. Occasional birds can retain a few to most upperwing secondary coverts, the outer greater coverts frayed, brownish, and with pale fringes (if not worn off), contrasting with replaced grayer formative inner and lesser coverts. These birds may also retain all juvenile primaries and some outer rectrices, relatively narrow, brownish, worn, and not showing molt clines indicating replacement. In most formative birds the outer primaries and inner secondaries are replaced in an eccentric pattern (see Molts); in these, typically 4–10 or more juvenile middle remiges (among s1–s4 and p1–p6) are retained, contrasting with fresher inner secondaries and outer primaries. Some may undergo complete Preformative Molts (see Molts) and cannot be reliably aged. Caution as well that older birds can suspend molt or have protracted molts, resulting in patterns resembling those of Formative Plumage following eccentric molts (cf. right-hand image below).

First Alternate Plumage

Present primarily April–September, although some individuals may remain in Formative Plumage through summer after lacking a First Prealternate Molt (see Molts). First Alternate Plumage otherwise variable, some birds formative-like but with a few fresher, rufous-fringed head and upperpart feathers or spotted breast feathers, but most birds resembling Definitive Alternate Plumage or nearly so; often these can only be aged by reatined inner primaries and outer secondaries when the wing is open. Molt limits among upperwing coverts and retained juvenile flight feathers continue as described under Formative Plumage but juvenile feathers become browner and quite frayed, including the outer primaries if retained. Often three generations of secondaries can be present, e.g., first-alternate tertials, one to many formative feathers distal to these, and the remaining outer secondaries juvenile. The rectrices may also contain three generations, with central feathers alternate, middle feathers formative, and outer feathers juvenile. Occasional to some individuals following complete Preformative Molts (see Molts) not separable from Definitive Alternate Plumage; outer primaries on these average fresher due to later replacement (20 Cramp, S., and K. E. L. Simmons (1983). The Birds of the Western Palearctic, Volume 3: Waders to Gulls. Oxford University Press, Oxford, UK. Close ) but ageing based on this alone not recommended.

Definitive Basic Plumage

Plumage generally rather plain. Crown is brown or slightly tinged reddish, streaked black. Upperpart feathers are brown with duskier centers or shaft streaks and narrow white fringing when fresh. The rump and central uppertail coverts are darker brown with little or no fringing; lateral uppertail coverts mostly white with dusky streaks forming characteristic appearance of most Calidris sandpipers in flight. Rectrices dusky brown with narrow white fringing when fresh, the central feathers darker. Sides of head with whitish supercilum and lower cheeks, streaked dark, a dark loral spot, and brown auriculars. Upperwing coverts and elongated tertials primarily brown with white fringing, the centers of the lesser coverts darker, toward blackish. Primaries and secondaries dusky gray with white shafts and very narrow white fringing when fresh. Underparts white except for brown wash and fine streaks across breast; flanks with diffuse narrow streaking. Underwing coverts variably white and blackish forming complex somewhat barred appearance (cf. ML574423391, ML190707411).

Definitive Basic Plumage separated from Formative Plumage (in most cases; see above) by basic upperwing coverts uniform in wear and quality, basic tertials uniform in wear within tract and with other secondaries, basic primaries uniformly darker, 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; some birds can suspend molt for migration and show more abrupt contrasts among these tracts but not as severe as those following eccentric molts in formative birds. 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 most body feathers replaced, the crown variably more rufous than in basic plumage, the upperparts variably fringed dull to bright rufous, and the underparts variably brown to cinnamon with distinct spots across the breast, changing to chevron-shaped marks ventrally and along the sides and flanks. The rump can be black, either from molt or the wearing of fringes from basic feathers. Upperwing secondary coverts with variable numbers of rufous-fringed alternate feathers, from a scattered few to most. Flight feathers similar to Definitive Basic Plumage except 1–4 tertials and 1–6 central rectrices can be alternate, contrasting with basic outer feathers. Ageing criteria similar to those between Formative and Definitive Basic Plumages; many first alternate birds can only be separated by retained inner primaries and outer secondaries as viewed on the open wing, and those following complete Preformative Molts not safely separable (see above).

Females and Males not known to differ in Definitive Alternate Plumage although brighter birds may well be males, as in other Calidris sandpipers, although plumage brightness may also vary by age; study on the breeding grounds is needed.

General

Molt and plumage terminology follows Humphrey and Parkes (26 Humphrey, P. S., and K. C. Parkes (1959). An approach to the study of molts and plumages. The Auk 76:1–31. Close ), as modified by Howell et al. (27 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. The 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 (28 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 (29 Jenni, L., and R. Winkler (2020). Moult and Ageing of European Passerines. Second edition. Bloomsbury, London, UK. Close ; see also 30 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 (30 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ). 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. However, for species that suspend prebasic or preformative molts for migration or undergo extensive molts on winter grounds, such as Sharp-tailed Sandpiper, there is often a lack of correspondence between H-P and life-cycle terms (30 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ). 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.

Sharp-tailed Sandpiper exhibits a Complex Alternate Strategy (cf. 27 Howell, S. N. G., C. Corben, P. Pyle, and D. I. Rogers (2003). The first basic problem: A review of molt and plumage homologies. The Condor 105:635–653. Close ), including a complete prebasic molt, a partial-to-incomplete (possibly sometimes complete) preformative molt, and a limited-to-partial prealternate molt (perhaps occasionally absent in the first cycle) in both first and definitive cycles (​​​ 20 Cramp, S., and K. E. L. Simmons (1983). The Birds of the Western Palearctic, Volume 3: Waders to Gulls. Oxford University Press, Oxford, UK. Close , 22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close , 25 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ). This, including protracted flight-feather molts on non-breeding grounds, has been termed a "Southern Hemisphere Molting Strategy" (pp. 500–505 in 25 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close ).

Prejuvenile (First Prebasic) Molt

Complete, late June–July, at natal site. No detailed information on timing or sequence of pennaceous feather irruption and development.

Preformative Molt

Partial to incomplete (possibly sometimes complete), primarily October through March or April. Preformative Molt occurs primarily or entirely at stopover sites or on the non-breeding grounds, not typically commencing on summer grounds except possibly for a few feathers occasionally (20 Cramp, S., and K. E. L. Simmons (1983). The Birds of the Western Palearctic, Volume 3: Waders to Gulls. Oxford University Press, Oxford, UK. Close ). It includes most to all body feathers and upperwing lesser coverts, some to most upperwing median and greater coverts, usually at least 1–4 tertials and central rectrices, and in most individuals, 3–6 outer primaries (among p5–p10) and corresponding primary coverts and 1–7 medial secondaries (among s5–s10, distal to the tertials) in eccentric sequence (see 22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close , 25 Pyle, P. (2008). Identification Guide to North American Birds. Part II: Anatidae to Alcidae. Slate Creek Press, Point Reyes Station, CA, USA. Close for details). Birds that undergo eccentric replacement usually replace all rectrices. Body feathers, tertials, and central rectrices are typically replaced in October–December whereas remaining flight feathers can be replaced in December or January–April, much later on average than primary replacement during Definitive Prebasic Molt; completion of Preformative Molt of primaries often overlaps the First Prealternate Molt (cf. 30 Pyle, P. (2022). Defining moults in migratory birds: A sequence-based approach. Journal of Avian Biology 2022:e02958. Close ). All primaries replaced on some birds (that retained 1–6 juvenile secondaries; 22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ) indicates that the Preformative Molt likely can be complete in some birds but study needed. It may be possible that some may retain juvenile primary coverts as in landbirds that undergo eccentric molts.

First Prealternate Molt

Limited (perhaps absent in occasional birds) to partial or incomplete, primarily April–June. Molt occurs primarily on non-breeding grounds but can complete at northbound stopover sites or on the summer grounds. Variable, may be absent in occasional birds, especially those that remain on non-breeding grounds during breeding season, but most appear to replace at least scattered upperpart feathers while others appear to have molt extent matching Definitive Prealternate Molt (see below). May average slightly later in timing than Definitive Prealternate Molt.

Second and Definitive Prebasic Molts

Complete, primarily July–November but molt of outer primaries extending to February in some individuals. 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. Molt of outer secondaries (proximally from s1) and central rectrices usually commences about when p6 is being replaced. Some body feathers and occasionally inner primaries (p1–p4, 22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ) can be replaced on summer grounds or at stopover locations during southbound migration, and usually completes or can occur entirely on non-breeding grounds, molt being suspended during southbound migrations between these locations. Most body feathers replaced October–December (occasionally retained into January) and flight feathers replaced primarily November-January (22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ).

Definitive Prealternate Molt

Partial, primarily February–May on non-breeding grounds but possibly can complete at stopover sites or on summer grounds in some individuals. It includes most to all body feathers, some to most proximal upperwing secondary coverts, usually 1–4 tertials, and usually 1–4 (occasionally to 6) central rectrices.

Based on descriptions in the literature (20 Cramp, S., and K. E. L. Simmons (1983). The Birds of the Western Palearctic, Volume 3: Waders to Gulls. Oxford University Press, Oxford, UK. Close , 21 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 , 22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ) and examination of Macaulay Library images.

Bill

In adults, the bill during the molting and non-breeding seasons is gray-brown darkening distally, with pale yellowish patch at base of lower mandible and often a small pale base to the upper mandible. During the prebreeding and breeding seasons a large proportion of the the base can brighten to olive, horn, or dull yellowish. In juveniles the bill is darker, blackish or resembling that on non-breeding adults. In anomalous individuals the bill can be yellowish to orange (e.g., ML74315631).

Iris

At all ages, blackish brown. It possibly averages duller or grayer in nestlings and juveniles.

Tarsi and Toes

As compared with other Calidris sandpipers and shorebirds,Sharp-tailed Sandpipers have relatively little webbing between the toes (cf. ML481337281, ML260726991). In adults, legs and feet vary from olive to greenish to yellow, appearing to brighten and becoming more yellowish during the prebreeding and breeding seasons. The legs and feet of juveniles may be yellow during the molting season of adults. Anomalous birds with bright orange or pinkish legs have been recorded (22 Higgins, P. J., and S. J. J. F. Davies (editors). (1996). Handbook of Australian, New Zealand & Antarctic birds. Volume 3. Melbourne: Oxford University Press. Close ).