Ruddy turnstones are small highly migratory wading birds. In all seasons, their plumage is dominated by a harlequin-like pattern of black and white. Breeding birds have reddish-brown upperparts with black markings. The head is mainly white with black streaks on the crown and a black pattern on the face. The breast is mainly black apart from a white patch on the sides. The rest of the underparts are white. The females are slightly duller than the males and have a browner head with more streaking. Non-breeding adults are duller than breeding birds and have dark grey-brown upperparts with black mottling and a dark head with little white. Juvenile birds have a pale brown head and pale fringes to the upperpart feathers creating a scaly impression.
Ruddy turnstones breed in northern parts of Eurasia and North America and fly south to winter on coastlines almost worldwide. Some non-breeding birds remain year-round in many parts of the wintering range, with some of those birds still taking on breeding plumage in the spring and summer. Ruddy turnstones can survive in a wide range of habitats and climatic conditions from the Arctic to tropical. They prefer to breed in open tundra with water nearby. Outside the breeding season, they are found along coasts, particularly on rocky or stony shores, and may venture onto open grassy areas near the coast. They are often found on man-made structures such as breakwaters and jetties. Small numbers sometimes turn up on inland wetlands, especially during the spring and autumn migrations.
Ruddy turnstones are social birds; they usually forage in small groups with other waders and gather at large roosts to sleep or rest. They feed by day using various behaviors to locate and capture prey. These behaviors can be placed into six general categories: Routing - when the turnstone manipulates piles of seaweed through flicking, bulldozing, and pecking to expose small crustaceans or gastropod mollusks hidden underneath; Turning stones - when the turnstone flicks stones with its bill to uncover hidden prey; Digging - when the bird creates holes in sand or mud with small flicks of its bill and then pecks at the exposed prey; Probing - is the behavior when the turnstone inserts its bill more than a quarter-length into the ground to get food; Hammer-probing - when the turnstone cracks open its prey's shell by using its bill as a hammer and then extracts the animal inside through pecking and probing; and the last feeding behavior is Surface pecking - when the bird uses short, shallow pecks to get at prey at or just below the ground's surface. When foraging, Ruddy turnstones adopt different postures indicative of their level of dominance. A lowered tail and a hunched stance is associated with chasing and aggression, and thus a dominant individual. Dominance in aggression is age-related, with juveniles assuming the subordinate role a disproportionate amount of the time. In order to communicate with each other Ruddy turnstones use a staccato, rattling call and also a chattering alarm-call which is mainly given during the breeding season.
Ruddy turnstones are carnivores and scavengers; their diet includes carrion, eggs, and plant material but they feed mainly on invertebrates. Insects are particularly important in the breeding season. At other times Ruddy turnstones also take crustaceans, mollusks, and worms. They may also prey on the eggs of other bird species such as gulls, terns, ducks, and even other turnstones.
Ruddy turnstones are monogamous and form pairs that may remain together for more than one breeding season. Their nest is a shallow scrape, often with a lining of leaves. It is about 11 cm (4.3 in) across and 3 cm (1.2 in) deep. It may be built amongst vegetation or on bare stony or rocky ground. Several pairs may nest close together. The female lays a single clutch of 2 to 5 eggs which are smooth, slightly glossy, and oval to pear-shaped. They are variable in color but are commonly pale green-brown with dark brown markings, densest at the larger end. Incubation begins when the first egg is laid and lasts for about 22-24 days. The female is mainly responsible for incubating the eggs but the male may help towards the end. The chicks are precocial and are able to leave the nest soon after hatching. They are buff above with dark grey markings and are white below. The chicks are able to feed themselves but are protected by the parents, particularly the male. They fledge after 19-21 days and become reproductively mature at 2 years of age.
Ruddy turnstones are very common and widespread throughout their range, however, they suffer nest predation and are susceptible to avian influenza. These birds are also vulnerable to climate changes and human disturbances during the nesting period.
According to the IUCN Red List, the total Ruddy turnstone population size is around 460,000-730,000 individuals, which roughly equates to 300,000-500,000 mature individuals. National population estimates include around 1,000-10,000 individuals on migration and around 50-1,000 wintering individuals in China; around 1,000-10,000 individuals on migration and around 50-10,000 wintering individuals in Taiwan, China; around 50-10,000 individuals on migration in Korea; around 1,000-10,000 individuals on migration and around 50-1,000 wintering individuals in Japan and around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in Russia. The European population consists of 5,900-77,100 pairs, which equates to 71,800-154,000 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red list site but its numbers today are decreasing.
Ruddy turnstones play an important role in their ecosystem as they are the main predators of various invertebrates which they consume their breeding grounds and crustaceans and mollusks in coastal habitats on their wintering grounds.