Sanderlings are small shorebirds that breed in the Arctic tundra and migrate long distances to wintering grounds on sandy beaches. Their mating behaviors and reproductive strategies are fascinating adaptations to their extreme environments.
Description of Sanderlings
Sanderlings (Calidris alba) are chunky, medium-sized sandpipers identified by their white plumage, black legs, and long black bill. Adults measure 6.3-8.7 inches in body length with wingspans around 14-16 inches. They typically weigh 1.3-3.2 oz.
Sanderlings have pale gray backs and wings in their non-breeding plumage. During breeding season, their upperparts become suffused with rich rufous coloration. Their bellies are white year-round. They constantly run and chase receding waves along shorelines, using their bills to rapidly probe for crustaceans, mollusks and other small invertebrates.
Breeding Range
Sanderlings breed in High Arctic areas of Canada, Alaska, Greenland, Iceland, Scandinavia and Russia. Nesting areas are located in barren coastal tundra habitats near wetlands or ponds. Sanderlings rely on open terrain with sparse vegetation for nesting. They favor desolate areas far from human disturbance and predators.
Wintering Range
After nesting, sanderlings undertake lengthy migrations up to 18,000 miles round trip. They fly enormous distances to reach wintering areas that offer mild weather and abundant food supplies along sandy beaches and estuaries. Wintering grounds are located along the Atlantic, Pacific and Gulf coasts of North and South America, western Europe, Africa, southern Asia, and Australia.
Physical Adaptations for Breeding
Sanderlings exhibit unique physical adaptations that aid survival and reproduction in the harsh Arctic tundra:
- Dense, insulating plumage to conserve heat
- Short wings that enable swift takeoffs for escape from predators
- Calcium reserves in bones to produce thick-shelled eggs
- Oil storing uropygial gland to waterproof feathers
- Large gizzard for digesting frozen foods
- Keen eyesight to locate insects and scan for danger
- Non-webbed feet to run across dry tundra and snow
Seasonal Plumage Changes
Sanderlings molt into breeding plumage before migrating to Arctic nesting areas. Their pale winter feathers are replaced with rich rufous feathers. The rufous coloration provides camouflage against the tundra environment.
After breeding, sanderlings molt again before fall migration. They revert to pale gray winter plumage. The pale coloration blends in with sandy beaches and makes the birds less visible to predators.
Courtship Displays
Courtship displays begin shortly after sanderlings arrive at Arctic breeding sites in late May or June. Males fly over tundra with slow, exaggerated wingbeats and issue melodic fluting calls to attract females. Displays reach a crescendo during peak nest initiation in early June.
Females crouch low to the ground with wings spread and quivering. This display stimulates males to land beside the female and begin ritualized postures. Males erect body feathers, expand the red throat patch and flick their wings. Pairs repeatedly circle one another while uttering harsh trilling calls.
Copulation
Once pair bonds are formed, females lead their mates on meandering chases over the tundra. At some point the female stops and crouches flat to the ground. The male promptly mounts her to copulate for a few seconds before falling off to one side. The female then stands up and resumes wandering.
No courtship feeding or parental care occurs. The male plays no role beyond contributing sperm. Some males mate with just one female per season, while others may mate with several different females. Females likewise often mate with multiple males during a single breeding season.
Nest Site Selection
Around one week after copulating, the female begins constructing a well-hidden nest in a shallow depression on open, elevated ground with good visibility. Ideal nest sites provide protective cover from predators and shelter from harsh weather.
Females build sparse nests by scraping out a shallow cavity and lining it with bits of tundra vegetation and leaves. Nests are typically placed near small stones, dried caribou dung or tufts of grass that help camouflage the nests in the open landscape.
Clutch Size
Sanderling clutches contain 3 or 4 eggs that are cryptically colored in brown, black, olive and purplish-gray. The eggs are heavily marked with dark blotches that aid camouflage. Eggs measure about 1.2 inches long and 0.9 inches wide. Female body reserves gained on wintering grounds provide resources to produce the eggs.
Incubation
Only female sanderlings incubate the eggs. Males depart the breeding grounds shortly after copulation. Incubation lasts for 21-26 days. To conserve heat, females develop incubation patches devoid of feathers on their belly skin. They diligently rotate and adjust the eggs.
During brief recesses from incubating, females leave the nest to quickly forage and replenish energy. Their cryptically colored plumage enables them to walk away from the nest unseen. They usually return within 30 minutes to resume incubation and warming the eggs.
Hatching
Sanderling chicks are precocial. They hatch covered in downy cryptic plumage and already capable of walking and feeding themselves. The chicks use egg teeth to pip out of their shells in unison over 1-3 days. Within 12 hours of hatching they leave the nest to search for food.
To avoid drawing predators, chicks disperse in different directions rather than stay together. The female broods them at night during their first week. But she provides no further care and soon abandons her offspring to begin molting and fattening for fall migration.
Development
Chicks forage entirely independently right after hatching. They use sensitive bills to probe for suitable invertebrate prey on the tundra. Chicks must feed constantly to fuel rapid growth and withstand harsh weather.
Within 5 weeks, juvenile sanderlings achieve flight capability. At 6-7 weeks maturity, juveniles begin migration south. Young birds migrate separately from adults. First year birds remain in wintering areas rather than returning to breed. Most individuals first return to Arctic nesting areas at age 2.
Sanderlings are relatively long-lived shorebirds. Lifespans in the wild can reach over 15 years. The oldest recorded sanderling was at least 17 years old.
Threats and Conservation
Sanderling populations currently appear stable. However, potential threats on their breeding and wintering grounds include:
- Habitat degradation from coastal development
- Human disturbance of nesting sites
- Expanding predator populations
- Oil spills and contamination
Conservation initiatives seek to preserve key wetland and beach habitats and limit disruptive activities near nesting areas when sanderlings are present.
Summary
Sanderlings are remarkably well-adapted to nesting in the High Arctic and wintering along the world’s coastlines. Their seasonal plumage variations, flexible mating strategies, cryptic eggs, and immediately independent young enable success across contrasting environments. Ongoing conservation efforts focused on protecting habitats and limiting disturbance will help ensure the continued survival of these globe-trotting shorebirds into the future.
