Curlew Sandpiper (Calidris ferruginea)

Curlew sandpiper
Adult on autumn migration starting moult out of breeding plumage, Pakistan
Non-breeding plumage, Thailand
Conservation status
Vulnerable  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Charadriiformes
Family:	Scolopacidae
Genus:	Calidris
Species:	C. ferruginea
Binomial name
Calidris ferruginea (Pontoppidan, 1763)
Range   Nonbreeding   Breeding   Migration
Synonyms
Tringa ferrugineus (Pontoppidan, 1763) Scolopax testacea (Pallas, 1764) Scolopax subarquata (Güldenstädt, 1775) Erolia variegata (Vieillot, 1816) Pelidna arquata (Brehm, 1855) Tringa chinensis (Gray, 1831) Erolia ferruginea wilsoni (Mathews, 1917)

The curlew sandpiper (Calidris ferruginea) is a small wader first described in 1763 by Erik Pontoppidan in the genus Tringa before being moved to its current genus, Calidris, in 1804 by Blasius Merrem. It is monotypic, with no subspecies recognised by any authority; however, it is known to hybridise with other waders occasionally. It is a long-distance migrant, breeding in the Siberian Arctic, arriving there from June and staying to August, migrating south throughout Europe and Asia, to winter mainly in Africa but also along the coasts of Australasia and Southeast Asia. It is a vagrant to North America.

They have three main plumages that change with maturity and season. The non-breeding plumage has brown-greyish upperparts, with the areas near the head and the breast being brighter. Their breeding plumage is much more striking, with the entire front side of the bird tinted a deep rufous, with the tint being stronger in males. First-year breeders do not migrate, instead remaining in their overwintering range throughout breeding season. However, they may occasionally still moult into their breeding plumage, which is largely similar to that of more mature curlew sandpipers but with white spots on the chest. The juvenile plumage is strikingly different, a soft peachy-buff on the breast, and with distinct pale fringes on the wing covert feathers, causing them to look 'scaly'. Adults superficially resemble the larger but shorter-billed red knot while breeding, and dunlin and stilt sandpiper while not breeding.

Their courtship behaviour is complex, with multiple different displays, including a ground display where the male makes a faux nest to present to the female, a graceful aerial chase accompanied with frequent trilling calls, and a precopulatory display, where the male would dance around the female, displaying his tail feathers and rump, which would sometimes be followed by copulation. They almost always lay one clutch of eggs, usually around the end of June, which hatch after 20 days, and fledge at about 14–20 days. They are omnivorous, foraging in wetlands in large flocks for various invertebrates, including crabs and insects.

The curlew sandpiper was formally described in 1763 by the Danish author Erik Pontoppidan under the binomial name Tringa ferrugineus.^[2] It was transferred to the genus Calidris in 1804 by the German naturalist Blasius Merrem.^[3][4] The genus name is from the Ancient Greek kalidris or skalidris, a term used by Aristotle for some grey-coloured waterside birds. The specific epithet ferruginea is from the Latin ferrugo, ferruginis, "iron rust", referring to its colour in breeding plumage.^[5] The curlew sandpiper is treated as monotypic, with no subspecies recognised.^[4] Within the genus Calidris, the curlew sandpiper is most closely related to the stilt sandpiper (Calidris himantopus).^[6][7] They occasionally hybridise with the sharp-tailed sandpiper and the pectoral sandpiper, producing the hybrids Cooper's sandpiper (Calidris × cooperi) and Cox's sandpiper (Calidris × paramelanotos), respectively.^[8][9]

The curlew sandpiper is a small wader, 18–23 cm (7–9 in) in length, 44–117 g (1.6–4.1 oz) in weight,^[10] and a wingspan of 38–46 cm (15–18 in),^[11] a bill length of 32–44 mm (1.3–1.7 in), and a tarsus length of 26–34 mm (1.0–1.3 in).^[12] They have black bills, dark brown irises, and dark grey to black toes and tarsi. All plumages have a large white patch on the rump, occasionally barred with red in some adults, and while in flight have a distinctive white wing stripe along the tips from the greater secondary to the greater primary coverts.^[a][13]

The plumage varies seasonally; the non-breeding plumage has upperparts that are initially greyish and underparts that are white with occasional large grey streaks; however, as feather wear increases, they will both slowly become more brown. The centers of their upperpart feathers are darker, especially on their crests, causing their upperparts to look mottled. The upper tail coverts as well as the lower rump feathers are white, with broad, pale grey rectrices. Their primaries and secondaries are dusky with white streaks along the shaft and a white tip. They have a distinctively white supercilium (a stripe going across their head) with grey-brown lores (the area around their eyes) and auriculars (the feathers protecting the ear). The post-breeding moult, taking place from July to November and lasting as late as April the next year for some southern populations, is a complete moult, with all feathers being replaced.^[13]

Occuring from January to May, the spring moult is a partial moult, with only the body feathers are replaced, not flight feathers. Returning breeders are sexually dimorphic. In females the front side, the entire head, and their upper back are tinted variably reddish, with grey-to-dark streaks. The lower scapulars (the feathers around the "shoulders" of the bird, where the upper wing meets the body), along with the upperwing coverts and commonly the tertials (the flight feathers closest to the body), are tipped ruddy-silver with black centers, resulting in a spotty appearance. The upper tail and upper wing coverts, along with the rump and flight feathers, are all nearly identical to their non-breeding appearance, except for the central rectrices and 2 to 3 tertials, which are off-black with white or ruddy fringing. Male plumage is similar to that of females, except with the rufous tinting being almost completely solid, and conspicuous spots above the eye and around the crown.^[13]

The pre-juvenile moult of chicks occurs at the nest site from late June to July. Juveniles look similar to the adult non-breeding plumage, except for more evenly spread-out wear marks and fringing, resulting in a scaly and more buff-tinged appearance. The crown and nape are both dusky with pale creamy streaks, the nape being slightly paler, and the rectrices tapered and narrower.^[13]

The post-juvenile moult, which occurs from October to December (and can finish as late as April), is a partial to incomplete moult, with the body feathers and some flight feathers are replaced. This moult takes place almost entirely at stopover and overwintering sites^[b], only rarely starting at breeding grounds.^[13]

The pre-breeding moult of yearling birds, which occurs from April to June, is absent or limited; the body feathers are largely or completely replaced, but few if any flight feathers are replaced. The first-time breeders plumage is similar to the adult summer plumage, with the primary differences being white spots or tufts on the underparts, which are fully, opaquely rufous, not tinted. Sex differences are also similar to that of returning breeders.^[13]

• 
  Juvenile plumage on autumn migration, Sweden
• 
  Adult moulting out of breeding plumage on autumn migration, St Petersburg, Russia
• 
  Adult in non-breeding plumage, South Africa
• 
  Adult in breeding plumage
• 
  A juvenile curlew sandpiper in flight. Note the white wing stripe and prominent white rump

Only males sing. Each song lasts for 10–15 seconds and consists of several parts, starting with several introductory notes, followed by multiple trilled doublets, a four-part phrase, and several drawn-out whine calls. Calls are uttered by both genders and include the following:^[14]

• Alarm call, a whik, whik, whik repeated or wik-ki-ki-ki... used on breeding grounds to warn of potential threats.^[14]
• Chase call, a shrill, musical twittering given primarily by the male but occasionally by the female during courtship flights.^[14]
• Flight call, a chirrup that drops in the middle, usually uttered during takeoff.^[14]
• Whine call, a loud and ascending whaay, whaay, whaay, nearly identical to the stilt sandpiper's whine and audible hundreds of metres away. It is only given by males and seems to be used for defending their territory.^[14]

While in breeding plumage, curlew sandpiper can be confused with red knot breeding plumage, as both are reddish on the belly. Compared to curlew sandpiper, the red knot is larger, has paler orange-buff underparts rather than rich brick-red; shorter, greenish legs, and with a stockier bill, as well as a spotted and greyish rump.^[13]

• Similar species while breeding
• 
  A male curlew sandpiper in full breeding plumage.
• 
  A red knot in breeding plumage.

The non-breeding or juvenile plumage looks similar to other species of their genus, Calidris, such as stilt sandpiper, which lacks the curlew sandpiper's distinctive white wing stripe and has longer, yellow legs and a flatter head, and dunlin, which has a much less distinct supercilium and a straighter bill, along with a generally stouter appearance, and wingtips that barely, if at all, extend beyond the tip of its tail. Juvenile curlew sandpipers can be distinguished from juvenile dunlin by the dunlin's distinctive brown or black spots on the sides of the upper belly.^[13]

• Similar species while non-breeding
• 
  An adult curlew sandpiper in non-breeding plumage; note the black legs
• 
  A stilt sandpiper in non-breeding plumage; note the yellow legs
• 
  A dunlin

In juvenile plumage, curlew sandpiper can be confused most easily with juvenile female ruff, which is similar colour though darker, and similar build but larger, while being shorter-billed. Dunlin is similar but slightly more compact shape and usually (but not always) shorter-billed, but can easily be distinguished by its streaked breast and flanks. Buff-breasted sandpiper is also a potential for confusion, but is much shorter-billed, and a more compact build.^[15]

• Similar species while in juvenile plumage
• 
  Juvenile curlew sandpiper; note the black legs, peachy breast, white flanks, and strong supercilium
• 
  Ruff in juvenile plumage; note the larger size, yellowish legs, darker underparts, and shorter bill
• 
  Dunlin in juvenile plumage; note the black streaks on the flanks and breast, compared to curlew sandpiper's pure white flanks and peachy-buff breast
• 
  Buff-breasted sandpiper in juvenile plumage; note the short bill, yellowish legs, and also the lack of a supercilium

The curlew sandpiper can be spotted throughout large swaths of Afro-Eurasia during migration. They are notably missing east of the Verkhoyansk Range in eastern Siberia during migration, along with southern inland Asia.^[13] They breed exclusively in the Siberian Arctic from the Yamal Peninsula to the Kolyuchin Bay.^[16] Their non-breeding range includes coastal regions of west Africa (from Gabon to Mauritania and including Cape Verde) and southern Africa (inland throughout Mozambique to Namibia to South Africa, and north through Uganda to Kenya). They are additionally present, albeit more rarely, inland from Niger to Chad, throughout the Republic of the Congo to Angola, and the Nile from Sudan to Egypt and southeastern Tunisia. They are much rarer along the coasts of Australasia as well as inland Victoria and New South Wales. In New Zealand, they are found on the southeastern coast of the South Island and the entire coastline of the North Island. They additionally overwinter in coastal South Asia as well as Southeast Asia. They do not overwinter in the rest of Eurasia. The curlew sandpiper is a common migratory vagrant in North America, especially along the Atlantic Seaboard.^[13]

During the non-breeding season, they are commonly found along sheltered mudflats, as well as salt pans and wetland edges. While inland they occupy the edges of various bodies of water, especially if they have muddy or sandy margins; they are also found along sewage ponds. They occupy coastal and inland wetlands during migration, and during breeding they mostly remain in lowland tundra near bogs, depressions, or pools, away from scrub and other dense vegetation.^[13]

Curlew sandpipers show little fidelity to breeding sites, making it hard to predict where a specimen overwinters based on its breeding site; however, the reverse is not as true: adults tend to prefer overwintering in the same regions and resting at stopovers at the same points, and males are more faithful to their sites compared to females. Both sexes have their breeding plumage by the time they arrive at their breeding sites.^[17]

Males depart from the breeding grounds when the female starts incubating, and females depart either when breeding fails or when the young successfully fledge. Fledglings migrate south soon after fledging; however, they do not migrate back for breeding during their first breeding season, instead remaining in their overwintering range.^[13][17] In western Europe (including Britain), post-breeding migration commences with adults arriving in July, and largely departing onward before juveniles pass through in August and September.^[18]

The curlew sandpiper appears to be largely monogamous; however, breeding habits are not well studied. Their behaviour is similar to that of other waders, walking, running, and wading to move around.^[13] Their flight speed during migration has been estimated to be around 70–75 km/h (43–47 mph).^[19] They practice preening and bathing and have been observed scratching their heads with their claws and dipping their heads in the water to clean them. Likewise, they mainly roost in large mixed-species flocks on sandspits, lagoon islets, and occasionally dunes, reefs, or beach wrack at high tide.^[13]

The curlew sandpiper forms pairs during northward migration.^[20] Pairs usually arrive at their breeding site in early June and perform courtship in mid-June. The female moves around an area about a few hectares large, feeding, and the male follows her attentively, trailing her 3–6 m (10–20 ft) away, occasionally giving partial or complete songs and conducting various types of courtship behaviour. Courtship behaviour includes the following:^[14]

• Aerial courtship, where the male pursues the female in long chases, occasionally catching up. When he catches up, he lifts his wings horizontally and glides toward the female while making trilling calls and occasionally performing his chase call. (The female also, although more rarely, performs the chase call.) The aerial chase closely resembles territorial conflicts, though the latter is more erratic, with sharper turns and gyrations. After the pair lands, the male then begins one of the ground courtship displays.^[14]
• Ground courtship attitude, where, during the pre-nesting period, the male intermittently assumes a "courtship" stance (lowered head, ruffled scapulars and back feathers, and bent legs, resulting in a hunched appearance) during feeding. It is often followed by a full ground courtship display and seems to represent increased libido in males.^[14]
• A nest-cup display, one of two ground courtship displays, is often preceded by courtship attitude behaviour. The male, in short bursts, moves across the tundra, eventually stepping into a small depression. Once in the depression, he settles down as if it were a nest, folding his wings over his back and raising his tail. He proceeds to wag his wings and tail side to side, occasionally crouching and kicking up lichen and moss with his claws. He then stands up, picks various bits of vegetation from around him, and deposits them by his feet in the faux nest. Even if the bit of vegetation slips, he still mimes the motion of tossing it into the nest. This sequence is typically followed by him stamping his claws or repeating the pressing motion from earlier, and during the sequence, he occasionally makes whine notes and trill calls. This display is not an act of nest-building, as the male performs it several times, each in different locations. Females are usually a few metres away but do not seem to pay full attention to the display.^[14]
• A precopulatory display. Described as elaborate, this display precedes copulation and begins with both members standing near each other. The male raises the wing closer to the female, reaching an angle of about 60°–70° with the horizontal, and raises the other wing to match it. He also raises his head and lowers and fans his tail. Following this, he gives a short series of trilling calls and sways back and forth, alternating between being behind her and in front of her. He attempts to display his fanned tail and rump to the female by keeping his back always facing her. The female, who has so far ignored him, dashes and thrusts her head toward the male's rump, occasionally overshooting such that her head ends up along the male's flank instead. After around a minute of this display, the female turns away, and the male, with his wings still held up high, approaches to mount and copulate.^[14]

The breeding grounds are occupied from June till late August.^[14] Nesting sites are usually located at the edge of a marsh or pool or on dry patches of tundra, often near nests of other curlew sandpipers. Clutches consist of 3–4 eggs (average 3.76) and are typically laid over the course of three days. The first brood is regularly laid from late June to July, with especially early breeders laying eggs only 5–6 days after arriving and the majority of pairs finishing around the end of June.^[13] A second brood may occur for some birds nesting in the far south of their breeding range if the first is unsuccessful.^[21] Eggs are slightly pear-shaped and olive-coloured with large dark olive or brown splotches, with more splotches on the broad end, giving them a roughly continuous shade of olive-brown. The female alone incubates the eggs for 20 days.^[13]

At birth, the nestlings are precocial (meaning relatively mature and mobile), covered with down, and begin to wander from the nest after drying. On average, they weigh 8.2 g (0.29 oz), with a bill length of 10.8 mm (0.43 in), a tarsus length of 23.2 mm (0.91 in), and a wingspan of 17 mm (0.67 in). The rate of growth is proportional to the temperature, with lower temperatures coinciding with slower growth and vice versa. The female leads the nestlings to good feeding habitats, and they become completely independent at 14–20 days of age. By the time they fledge, they weigh on average 49.6 g (1.75 oz), with a bill length of 27.8 mm (1.09 in), tarsi 30.8 mm (1.21 in), and a wingspan of 94.5 mm (3.72 in).^[22]

Territorial marking and disputes are uncommon in the curlew sandpiper, with reports of pairs displaying complete disregard for the territories of others; however, those areas were sparse and newly settled and thus did not need to be defended. In more densely populated areas, curlew sandpipers are territorial, chasing off other birds including dunlin, Baird's sandpiper, and pectoral sandpiper. Territories varied from 1.6–4.0 hectares (4.0–9.9 acres) in size for pairs. Territorial behaviour included:^[14]

• Restricting their activity to their territory, with aerial displays and 'whine' notes to announce territory.^[14]
• Chasing of intruders up to the edge of their territory, in flight or on the ground, similar to the aerial courtship display except more erratic, jagged, and rapid. The ground aggressive display has only been observed once and consisted of the male lowering the head and neck parallel to the ground, pulling back towards the body, and fanning his tail before running towards the invading bird.^[14]
• Stopping chase at the boundaries of their territory, suggesting awareness of their neighbor's territory. Upon concluding a chase, they would often perform a flight song while returning.^[14]

The curlew sandpiper, like other waders, forages in large flocks of up to thousands on tidal flats, shallows, marshes, salt flats, and sewage lagoons, picking up food by sight,^[13] only rarely swimming while feeding.^[23] They are active throughout the entire day and night; however, the frequency at which they forage at night decreases as the time for northward migration approaches. They eat insects and other small invertebrates like crabs, molluscs, and worms, supplemented with various seeds.^[13]

The curlew sandpiper has an estimated annual survival rate of 75–80%, with the oldest observed individual being 19 years old.^[24] Information on their lifespan and breeding success are not well researched.^[13] The reproductive success of this species is strongly related to the population of lemmings, specifically the West Siberian, East Siberian and Arctic lemmings. In years with fewer lemmings, predatory species such as the Arctic fox would hunt Arctic-breeding waders including the instead.^[10][25][26]

They are subject to infestations from parasitic worms, with a total of 154 worms collected from five curlew sandpipers as part of a migratory bird study in Tasmania. Nadejdolepis paranitidulans was the most common parasite at 87% of all collected. A total of five species of parasitic worms were observed, with four cestodes and one trematode.^[27]

Curlew sandpiper counts in South Africa at Langebaan Lagoon, where they are most numerous, show a 40% decline in numbers between 1975 and 2009. A similar trend has been observed in Australia and may be linked to the effects of global warming at the artic breeding grounds.^[28] They have an extremely extent of occurrence, around 3,000,000 kilometres (1,900,000 mi), have an estimated population of 420,000 to 960,000 mature individuals. BirdLife International has judged the species to be vulnerable, as over the span of 15 years, the population has declined and estimated 30 to 49%.^[1] The curlew sandpiper is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies.^[29]

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