Least Sandpiper

On the breeding grounds, the Least Sandpiper feeds mainly on the larvae of midges, chironomids, and other flies, supplemented by other small insects. During coastal migration stopovers, the diet shifts toward small crustaceans — particularly amphipods and isopods — as well as small snails, marine worms, and other invertebrates. In spring on the Atlantic Coast, it joins other shorebirds in feeding on the eggs of horseshoe crabs, a rich energy source that fuels the final push to the breeding grounds. Inland migrants eat mostly insects. Seeds of marsh grasses such as smartweed and panic grass also feature in the autumn diet.

The species uses two main foraging techniques: picking prey items from the surface, and probing damp mud with its bill for buried prey. Females, with their longer bills, probe deeper and access larger, more deeply buried prey items than males. This means the two sexes effectively partition the food resource on shared feeding grounds, reducing competition between mates.

A feeding mechanism published in Science in 2008 revealed that while probing damp mud, the sandpiper uses the surface tension of water surrounding a prey item to transport it from the bill tip to the mouth in a stepwise "capillary ratchet" motion. The bird rapidly opens and closes its bill in a tweezering action, moving a water droplet containing the prey toward the mouth in incremental steps — a physical feeding mechanism previously unknown in birds. This mechanism requires no muscular effort beyond the bill movements themselves. It also has a conservation implication: surface pollutants such as oil or detergents that alter water's wetting properties can disrupt the ratchet entirely, potentially rendering affected birds unable to feed efficiently.

The Least Sandpiper forages in a characteristic crouched posture, walking slowly with its head lowered and body held nearly horizontal — a hunched, almost furtive gait that sets it apart from the more upright stance of larger waders. It picks prey from the surface and probes damp mud with its bill, often working the upper edges of mudflats where the substrate is slightly drier than the wet mud favoured by Semipalmated and Western Sandpipers.

This preference for higher, drier microhabitats within wetlands is a consistent behavioural trait. On coastal mudflats, Least Sandpipers are typically found at the upper margins, often partially within fringing vegetation, while other small sandpipers work the wetter, lower zones. This habitat partitioning reduces direct competition with closely related species.

Outside the breeding season, the species is generally gregarious, forming loose flocks during migration and on wintering grounds, often associating with other small sandpipers in mixed groups. On the breeding grounds, however, males are strongly territorial, defending areas against rival males with aerial display flights and ground-level threat postures.

When disturbed, birds often crouch and freeze before flushing, relying on their cryptic brown plumage to avoid detection. Flushed birds typically fly low and fast before dropping back into cover a short distance away.

The most frequently heard call is a high-pitched, rising "kreep" or "greeep" — a clear, reedy, rolling note that carries well across open wetland. This flight call is given regularly during migration and is one of the most reliable ways to detect the species, particularly when birds are moving overhead at night. When feeding, birds also give a soft, low "prrrt" or chuckling note during close-range interactions.

On the breeding grounds, unpaired males produce three main categories of vocalisation. The first is a series of Rhythmically Repeated Calls used to attract mates and in agonistic interactions with rival males. The second is a sustained aerial song delivered during display flights — a monotonous, single repeated note given continuously as the male circles the territory. The third is a chatter call used in close-range encounters. Research into geographic variation in vocalisations found no differences between geographically separated populations, suggesting little or no vocal learning in this species — an unusual finding among songbirds and waders alike.

In flight, the Least Sandpiper appears very small and compact, with relatively short, pointed wings and a fast, direct wingbeat. The flight action is typically rapid and slightly erratic over short distances, with birds often twisting and jinking before dropping back into cover. Over longer distances — particularly during migration — flight becomes more direct and purposeful.

The key features to look for in flight are the narrow white wingbar, visible against the dark upperwing, and the white sides to the rump and uppertail bisected by a dark central stripe. This rump pattern is shared with several other small sandpipers but, combined with the small size and warm brown tones, helps narrow identification. The underwing is pale.

On migration, birds often fly in tight, fast-moving flocks that twist and turn in unison — the classic "peep" flock behaviour. Flocks frequently call in flight, and the high "kreep" call is often the first indication of the species' presence overhead, particularly during nocturnal migration. The species is a long-distance migrant capable of extraordinary sustained flight: eastern populations undertake nonstop transoceanic crossings of up to 4,000 km, one of the longest overwater flights made by any shorebird of this size. Western populations follow a different strategy, making shorter flights with more frequent stopovers through the interior of North America or down the Pacific Coast.