Western Sandpiper

Males arrive on the Alaskan breeding grounds from late May, often contending with snow and ice for several days before the tundra clears. They immediately establish territories of 0.5–3.5 acres, performing display flights and calling persistently. Females arrive shortly after and are initially unreceptive, preferring to feed and rebuild energy reserves after the northward migration. Full details of courtship displays are covered in the courtship and display section.

The species is monogamous, with a strong pair bond throughout the breeding season.

The nest is a shallow scrape in flat, dry tundra, typically beneath a grass tussock or dwarf birch, lined with dry willow and birch leaves, grasses, sedges, and lichens. The interior cup averages about 6.4 cm across and 5.6 cm deep. The clutch typically contains 4 eggs (range 2–5), white to cream or brown and densely spotted with brown, measuring approximately 2.94–3.32 cm long and 2.14–2.35 cm wide.

Both parents share incubation over approximately 21 days (range 20.5–22 days). Most clutches hatch around mid-June. Hatchlings weigh 4.4–5.4 g and are precocial — active and downy, leaving the nest within hours and foraging independently within 24 hours. The female typically departs before fledging, leaving the male to complete chick-rearing alone. Young fledge at 17–21 days. Many first-year birds that winter in Central America do not return to the breeding grounds in their first spring, reaching sexual maturity at around one year old.

The maximum recorded lifespan for a wild Western Sandpiper is 9 years and 2 months, established through a banding record in Kansas (Cornell Lab / USGS Bird Banding Laboratory). Typical lifespan in the wild is estimated at 5–9 years, though annual survival rates mean many birds do not approach this maximum.

Annual survival is influenced heavily by conditions at a small number of key stopover sites. Because so much of the global population concentrates at the Copper River Delta and a handful of other locations, a single adverse event — an oil spill, a severe weather event, or a disease outbreak — could affect survival rates across a significant proportion of the population in a single season.

Predation is a significant mortality source throughout the annual cycle. Peregrine Falcons are the primary aerial predator during migration, with predation pressure increasing steeply from late July as peregrines return from northern breeding areas. On the breeding grounds, Arctic foxes, jaegers, and other tundra predators take eggs and chicks. Many first-year birds that winter in Central America do not return to the breeding grounds in their first spring, suggesting that first-year survival or the energetic cost of the full northward migration is a limiting factor for young birds. Compared to the Wood Sandpiper, which has a maximum recorded age of around 11–12 years, the Western Sandpiper's banding record is somewhat lower, though both species face similar long-distance migration pressures.

The Western Sandpiper is listed as Least Concern on the IUCN Red List (2016), with a global population estimated at approximately 3.5 million mature individuals by BirdLife International and Partners in Flight. Despite this large total, the population trend is decreasing, and the species scores 12 out of 20 on the Partners in Flight Continental Concern Score — indicating low but monitored concern.

The single greatest structural vulnerability is the species' extreme dependence on a tiny number of stopover sites. Because nearly 80% of the global population concentrates at the Copper River Delta within a few weeks each spring, any significant habitat degradation there — from an oil spill, pollution event, or sea-level rise — could have population-level consequences in a single season. The Fraser River Estuary in British Columbia is similarly critical, regularly hosting 14–21% of the Pacific Flyway population.

Specific threats include: wetland drainage across Middle America for shrimp farms and agriculture, which reduces wintering habitat; the spread of invasive smooth cordgrass (Spartina alterniflora), which has heavily invaded estuaries in Washington state and California, smothering the intertidal mudflats on which foraging birds depend; oil spills and chemical pollution at coastal stopover sites; and climate change, which threatens intertidal mudflat habitats through sea-level rise, alters Arctic food webs, and is already shifting migration timing. Human disturbance from recreation, aquaculture, and coastal development at key sites adds further pressure.

Conservation responses include the Western Hemisphere Shorebird Reserve Network (WHSRN), which has designated key stopover and wintering sites as reserves, and ongoing population monitoring programmes. Research into biofilm ecology has also highlighted the importance of maintaining the full intertidal gradient at stopover sites — not just the mudflat surface, but the microbial communities that coat it.

Male Western Sandpipers arrive on the Alaskan tundra before females and begin establishing territories immediately, often while snow still covers much of the ground. Territorial advertisement involves two aerial display flights performed repeatedly over the breeding territory: the "butterfly flight", in which the male flies slowly with deep, exaggerated wingbeats while singing, and a fast, low, swooping flight that ends with an abrupt upward pull. Both displays are accompanied by the territorial song — a complex series of trills and buzzy notes that carries across the open tundra.

Ground displays are equally elaborate. When a female is present, the male approaches sideways in a hunched posture: wings drooped and trembling, tail cocked upward, head lowered. He gives a distinctive trilling "brrrt, brrrt" call throughout this approach. Wing-up and tail-up postures are also used. Females are initially unresponsive, preferring to feed, but after several days begin to follow males to potential nest scrapes. The male prepares multiple scrapes across his territory; the female selects one and signals acceptance by crouching and raising her tail, after which copulation occurs.

The pair bond is strong and monogamous throughout the breeding season. Approximately half of returning adults re-pair with their previous season's mate — a level of mate fidelity that is relatively high for a small shorebird. Despite this, up to 8% of chicks may result from extra-pair copulations, suggesting that mate-guarding is imperfect. Once the clutch is complete, both parents share incubation, but the female's investment decreases as hatching approaches, and she typically departs the breeding grounds before the chicks fledge — leaving the male as sole carer for the final days of chick development.

Few shorebirds show as clear a link between physical dimorphism and ecological separation as the Western Sandpiper. Females average approximately 15% longer bills than males — a difference large enough to be visible in the field on well-seen birds. Female bills are also more strongly drooped at the tip, enabling deeper probing into intertidal mud. This allows females to access buried prey items that males cannot reach, reducing direct competition between the sexes on shared foraging grounds.

The ecological consequences extend far beyond the mudflat. Across the wintering range, the sexes are geographically segregated by latitude: males predominate in the northern parts of the wintering range, making up 70–80% of populations in California, while females predominate in the south, with females outnumbering males in Ecuador and other equatorial wintering areas. This latitudinal sex segregation is thought to be driven partly by bill morphology — longer-billed females can exploit the productive, fine-grained intertidal habitats of more southerly estuaries — and partly by the earlier departure of females from the breeding grounds, giving them a head start on the southward journey.

The size difference also extends to body mass: females are slightly heavier than males overall, and depart the breeding grounds earlier, arriving on the wintering grounds ahead of males. This means that at any given wintering site, the sex ratio shifts through the season as males arrive later. The interaction between bill length, foraging niche, migration timing, and wintering latitude makes the Western Sandpiper an unusually well-studied example of how sexual dimorphism can drive large-scale ecological separation within a single species.