Least sandpiper

Calidris minutilla is a shorebird of the family Scolopacidae, described by Vieillot in 1819 under the name Tringa minutilla ^[1]. It is the smallest of the Nearctic Calidris; it nests in northern North America, from the boreal forests to the subarctic tundra, and moves south outside the breeding season ^[2]. In Peru it is called «playerito menudo» ^[3]. The IUCN Red List classifies it as Near Threatened in its 2024 assessment ^[4].

It is a very small sandpiper. In Buenos Aires, a bird standing next to a Calidris fuscicollis was easy to identify by its smaller size and its short, yellowish-green legs with fairly long toes; it was feeding by probing the mud with its bill ^[5]. Females are somewhat larger than males, and in birds caught from California to Ecuador both wing and bill were longer in those that winter farther south ^[6].

A single measurement is not enough to sex birds from their dimensions. In 110 birds collected in Cuban wetlands and sexed by dissection, a function combining culmen length and wing length assigned sex correctly to 91 % of individuals, whereas the method based on bill length alone was right in only 44 % of cases and was biased towards females ^[7].

Similar species

• Calidris fuscicollis: Calidris minutilla is smaller and has short, yellowish-green legs ^[5].

Outside the breeding season it lives mainly from the south of the United States through Central America and the Caribbean to the north of South America; in Argentina it is a rare visitor ^[5]. In Surinam it is a numerous migrant, most abundant from mid-August to early May, and a few birds remain there through the northern summer ^[8]. Females tend to winter farther south than males ^[6].

In winter it uses lakes, marshes, mudflats, flooded farmland, drainage ditches and coastal salt marshes, among other settings ^[2]. At the Caucato wetland in Pisco, most of the birds counted were on the lagoon shore (91 of 164) and in the rush beds (43) ^[9]. At Poza de La Arenilla in La Punta (Callao), it was very scarce in the 2013 to 2015 surveys, and the study's species list marks it only in spring ^[10].

On Sable Island, Nova Scotia, near the southern edge of its breeding range, males are the first to arrive in mid-May, and clutches are completed between late May and late June ^[11]. Eggs are laid at a mean interval of 1.2 days, incubation lasts 20 to 21 days, and mortality of nests and chicks is high, mainly through predation by the herring gull ^[11].

Before pairing, males fly over the site where they will nest at about 10 to 30 m, alternating glides with bursts of rapid wingbeats and repeating a call about 2 times per second; in Nova Scotia these flights lasted 3.4 minutes on average ^[12]. On Sable Island it was strictly monogamous: males tended to incubate by day and females by night ^[13].

Chicks fly well at 14 to 16 days of age, but the female stays with them for about 6 days on average and the male for about 20 ^[13]. On the Queen Charlotte Islands, British Columbia, broods of different pairs were regularly seen to merge, and adults to care for chicks not their own, though at a low rate ^[14]. During the 2002 autumn migration in the lower Mississippi valley, 293 colour-marked birds were followed: the estimated stay at each stopover site was 4.1 to 6.9 days depending on site and age, and body condition explained little of that length of stay ^[15].

It eats mostly small invertebrates, and on its wintering grounds its diet is more varied than on its breeding grounds. In organic rice fields in the Valle del Cauca, Colombia, 50 birds were caught between October and December 2012: 30 regurgitates containing food held 1410 items from 22 groups, and insects were by far the best represented (91 %), followed by plant remains (8 %), such as rice seeds and grass stems, and spiders (1 %) ^[2]. These birds swallowed 221 small stones of 0.01 to 1 mm that help grind hard prey such as beetles, and the authors suggest the species helps to control beetles that are rice pests ^[2].

At two wetlands in Cuba it made up more than 70 % of the mixed flocks it shared with Calidris pusilla and Calidris mauri, and in birds collected there in October 2008 its diet was dominated by fly eggs, larvae and pupae ^[16]. On the coast of Ecuador it was found carrying the chewing louse Actornithophilus umbrinus, which also occurs on other migratory shorebirds ^[17].

Its global category is Near Threatened ^[4]. It depends on wetlands along its whole route: in the St. Lawrence River estuary, Canada, during the autumn migrations of 2011 and 2012 the probability of finding it rose with the width of the upper salt marsh; the authors note that salt marshes are vulnerable to sea-level rise and that in this estuary they can no longer shift inland, so they call for careful management of the shoreline ^[18].

At the Caucato wetland in Pisco, vehicle noise, people and domestic animals made the birds go on alert, shift position or take flight within the wetland ^[9].

In the north it appears on the fauna list of the Los Manglares de Tumbes National Sanctuary, in its 2023 management plan ^[3]. On the southern coast, at the Caucato wetland in Pisco, 164 individuals were counted in surveys from 2017 to 2019, 5.12 % of the birds recorded ^[9]. On the seashore of San Andrés, also in Pisco and in the northern part of the buffer zone around the Paracas National Reserve, 50 birds were counted in January 2023 and 12 in December 2024, on sandy, stony and rocky beaches ^[19].