Semipalmated Sandpiper (Calidris pusilla)

[order] CHARADRIIFORMES | [family] Scolopacidae | [latin] Calidris pusilla | [UK] Semipalmated Sandpiper | [FR] Becasseau semipalme | [DE] Sand-Strandlaufer | [ES] Correlimos semipalmeado | [NL] Grijze Strandloper

Subspecies

Physical charateristics

The Semipalmated Sandpiper Calidris pusilla, only about 30 g in weight and 14 cm long, is one of the smallest shorebirds. Its name comes from the partial webs between its toes. Males and females are identical in rather plain brown or grey plumage with white underparts although females are slightly larger. The legs are black. The species can be difficult to distinguish from other small sandpipers.

Semipalmated Sandpipers moult, or shed, their body feathers twice a year. The change to the greyish-brown fall-winter plumage usually starts on the breeding grounds and is completed after arrival on the non-breeding area. The moult that takes place on the non-breeding area prior to spring migration gives them a slightly brighter (more brown) breeding plumage. Adults moult their flight feathers (wings and tail) gradually-retaining the ability to fly at all times-and only once per year, usually in the non-breeding area.

Some juveniles do not replace any flight feathers in their first winter, as these are quite new. Others, however, moult some of the outermost primaries (outer wing feathers), which are important for flight and wear most rapidly.

Listen to the sound of Semipalmated Sandpiper

[audio:http://www.aviflevoland.nl/sounddb/S/Semipalmated Sandpiper.mp3]

Copyright remark: Most sounds derived from xeno-canto

Range

North America : North

Habitat

Habitat in breeding range, low and sub-arctic tundra, near water. Found nesting in river deltas in dry shrubby areas of mixed sedges and grasses. Variably drained upland tundra with low vegetation near small ponds, lakes, and streams or moist, wet sedge-grass or heath tundra Sandy areas along rivers and pond-dotted sand dunes.

Stages (stopover) during migration in areas of shallow fresh or salt water and little vegetation, muddy intertidal zones, or along edges of lakes, usually on soft silt/clay mudflats, or at junction of short-grass marsh and tidal flats. During winter
generally areas of shallow lagoons with dead mangroves; also low tidal zone of mudflats on wet or dry mud.

Reproduction

Males usually arrive on the arctic breeding grounds in late May, a few days before females. Each male attempts to obtain a territory-often the same one as in previous years-and to attract a female by flight displays, hovering in the air while uttering sharp cries. Some pairs breed together for three or four years, although they apparently do not spend the winter months together. It is uncommon for both birds to return but find different mates. Like most small sandpipers in Canada, Semipalmated Sandpipers are monogamous, each bird having only one mate at a time.

Once the pair is formed on the breeding grounds, the male makes small depressions, or scrapes, in the ground. The female chooses one of the scrapes and lays eggs in it when she has obtained enough food to produce them. Most birds lay their eggs in mid- to late June, very few as late as early July.

The female almost always lays four eggs, usually one per day. These eggs are very large, together weighing almost as much as the female. They are “pyriform” in shape, or quite pointed at the small end. This allows them to fit together tightly in the nest cup, which keeps the eggs warm longer, an important factor in an arctic environment. The eggs are well camouflaged, being speckled and splashed with dark brown and olive colours against a light green or brown background.

Once the clutch, or set of eggs, is complete, the parent birds share the task of incubating, or keeping the eggs warm until they hatch. This takes about 19 days. Many eggs are lost to predators, such as gulls, hawks, and foxes. Semipalmated Sandpipers are too small and defenceless to attack predators and instead rely on distraction displays to decoy them away from the nest area. Egg predation is higher in years when mice and voles are rare (every three or four years in the Arctic), because predators that normally exist on small rodents switch to other prey, such as shorebird eggs and young.

Weather may also be a problem for breeding Semipalmated Sandpipers. If the insects and tiny water animals that they feed upon are unavailable because of high water levels or extremely low temperatures, incubating birds may be forced to desert their nests to survive. In some years, many females are unable to lay eggs at all, probably because they cannot obtain enough food.

Because the eggs are not incubated until the clutch is completed, all four young normally hatch within 24 hours of one another. Chicks are extremely independent right from birth. They are born with open eyes and almost adult-sized legs. As soon as they are dry, they begin to stumble about, pecking for insects. Chicks are not fed by their parents at all but are periodically brooded, or kept warm by a parent, during their first week. They begin to fly weakly at about 14 days of age and can fly fairly well at about 18 days.

Females normally desert their broods within 10 days after they hatch, leaving them to the care of the male. The later in the year the nest hatches, the earlier the female leaves. This is thought to result in a higher survival of females, which may still be stressed from egg laying. Males usually desert the chicks at about the time the chicks fledge, or take their first flight.

Feeding habits

Semipalmated Sandpipers feed on insects and tiny, mostly shrimplike, water animals that collectively are called aquatic invertebrates.

Conservation

This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Migration

The sandpipers breed in sub- and mid-arctic areas of Canada and Alaska. They begin their northward migration from South America in early to mid-May each year. Some juveniles do not migrate north in the spring or attempt to breed until their second year. However, replacement of the outer primaries, or outer wing feathers, may enable those that do attempt to breed in their first year to reach the breeding grounds successfully. Little food is available on the eastern coast of Canada in the spring, so this area is important only during fall migration. Migration northwards is relatively rapid. Flocks of up to 350,000 birds may gather at key stopover sites. About two thirds of juveniles spend summer on wintering grounds. High fidelity to staging areas and breeding grounds, but unknown for wintering grounds.

The migration south begins in early July, when failed breeders and nonbreeders start out, followed quickly by adult females and then males that have left their young. Peak migration of adult Semipalmated Sandpipers in populated areas of Canada occurs around the end of July or beginning of August. Juveniles peak several weeks later; some travel with late adult migrants, but many apparently find their way to the wintering areas by instinct.

Most Semipalmated Sandpipers from the western Arctic migrate south through the interior of North America. Many of those that nested in the central or eastern Arctic stage in southern James Bay, the St. Lawrence estuary, and the Bay of Fundy in Canada, as well as on the northeastern coast of the United States. Instead of following the eastern coast of North America, most birds fly nonstop to South America over the Atlantic, a distance of more than 3 000 km.

Distribution map