Semipalmated Sandpiper

Calidris pusilla is a shorebird of the family Scolopacidae whose valid name is attributed to Linnaeus (1766) ^[1]. It breeds in the Arctic, from Alaska eastward across the Canadian tundra, and spends the non-breeding season in regions of South America; Paracas, in Peru, lies at the southern edge of that range ^[2]. The IUCN Red List classifies it as Near Threatened ^[3]. In French Guiana, the centre of its wintering range, the wintering population fell by about 80% between 1980 and 2008-2010 ^[4].

The valid name of the species is Calidris pusilla, with authority Linnaeus (1766) ^[1]. WoRMS also lists Tringa pusilla and Ereunetes pusillus, both by Linnaeus (1766), as unaccepted synonyms ^[1].

Bill length (culmen) varies along the breeding range: western populations have a shorter bill on average than eastern ones ^[2]. At Paracas, culmen lengths cover that whole interval, with a slight lean toward short bills, and the overall mean is about 18 mm ^[2].

Juveniles of the year are seen at Paracas from September and are recognised by plumage, above all by the juvenile-type inner greater coverts they still retain ^[2]. Adults are recognised by the shape and colour of freshly moulted primaries: they moult completely on arriving at the non-breeding area or shortly before ^[2]. Juveniles travel south a month after the adults and do not moult on arrival, although some replace 1 to 6 primaries from January to March ^[2].

It breeds in the Arctic, from Alaska eastward across the Canadian tundra, and every year it travels between those areas and the regions of South America where it spends the non-breeding season ^[2]. On autumn migration its route includes a flight of about 5300 km over the Atlantic Ocean, and about 75% of the world population stops over in the Bay of Fundy, in Canada ^[5].

Wintering sites include the coast of Suriname and Canela Island, in northern Brazil ^[6],[5]. The aerial counts of French Guiana were made in the centre of the wintering range ^[4]. Paracas, in Peru, is at the southern edge of that range, and several thousand birds spend the non-breeding season there ^[2].

At Paracas it uses an intertidal mudflat about 2 km long and 50 m wide, surrounded by coastal desert, which a 1.5 m tide covers twice a day ^[2]. In the Caucato wetland, in Pisco, 222 individuals were counted, 155 of them on the lagoon shore ^[7]. On the sandy and stony beaches of San Andrés, also in Pisco, it is listed as a Nearctic migrant ^[8].

Some of the birds at Paracas do not make the migration to the Arctic and spend the boreal summer there. A mark-recapture model using data from October 2014 to March 2019, based on 1963 marked birds, estimated that 28% of one-year-old birds and 19% of adults stay ^[2].

Birds that stay survive better than those that migrate. From April to September, the estimated survival of adults was 0.89 if they stayed and 0.68 if they migrated, and that of one-year-old birds was 0.81 and 0.67 ^[2]. In one-year-old birds that migrate, the calculated survival depends on distance: it is high and steady with culmens longer than about 19 mm and drops quickly below that length, a result the authors regard as tentative ^[2].

At La Pérouse Bay, in Manitoba, based on 213 nests measured from 1980 to 1982, the mean clutch of adults was 3.9 eggs, with 77% hatching, and that of one-year-old birds was 3.8 eggs, with 44% ^[4].

On spring migration, shorebirds make a stopover of 2 to 3 weeks in May in Delaware Bay, where they eat large numbers of the eggs of the horseshoe crab Limulus polyphemus ^[6]. Stopovers serve to build up fat before long flights without pause ^[5].

Ydenberg and Hemerik report the proposal that the Peregrine Falcon (Falco peregrinus) has made migration more dangerous: after introductions made after 1975, pairs now breed at Atlantic staging areas, such as the Bay of Fundy, that once had no breeding peregrines ^[4]. According to that proposal, two large, independent studies found that the survival of this species during migration, from April to September, is 0.15 to 0.17 lower than that of Calidris mauri ^[4].

The IUCN Red List classifies it as Near Threatened ^[3]. In French Guiana, the aerial counts of 1980 were repeated in 2008-2010 with the same observer and the same method, and showed a fall of close to 80% in the wintering population ^[4]. Censuses of breeding birds in the Arctic and trends at staging sites also indicate a large decline, although a growth estimate based on western Arctic breeding populations does not differ significantly from a stable population ^[4].

A demographic model based on the Paracas data (28% of one-year-old birds and 19% of adults staying) calculated that, if those rates had risen from 10% and 5% and the population had been stable before, it would have fallen 42.5% in 20 years; the authors present this as a hypothesis that deserves further study ^[4].

The Paracas study was carried out inside the Paracas National Reserve ^[2]. The beaches of San Andrés, in Pisco, lie at the northern edge of its buffer zone and are strongly influenced by people along their whole length ^[8].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In Suriname, blood was taken from 71 birds captured between 23 and 28 April 2013. Second-year birds had more cadmium, cobalt and lead than older ones, and there were no differences by age in arsenic, mercury or selenium ^[6].

Adults in Suriname had more cadmium, mercury and selenium in the blood than those in Delaware Bay, in New Jersey (mercury, mean 17 ppb in Suriname and 12.7 ppb in New Jersey), and those in New Jersey had more arsenic ^[6]. The authors recall that illegal gold mining has spread mercury contamination in Suriname, and point out that selenium may be in a worrying range, although little is known about the levels that harm wild birds ^[6].

In Peru, the best-studied site is La Aguada beach, in the Paracas National Reserve, where birds were marked and observed between October 2014 and March 2019 and several thousand individuals gather ^[2]. On the beaches of San Andrés, in Pisco, 391 birds were counted in the January 2023 census and 29 in the December 2024 one ^[8].

In the Caucato wetland, in Pisco, 222 individuals were recorded in counts made in 2017, 2018 and 2019 ^[7]. At Poza de la Arenilla, an artificial wetland of about 18.2 ha in La Punta, Callao, the highest values were 66 and 64 individuals, in the winters of 2014 and 2016, during a study from 2013 to September 2016 ^[9].