Curlew Sandpipers Calidris ferruginea on their breeding grounds: schedule and geographic distribution in the light of their breeding system

Curlew Sandpipers arrive on their breeding grounds in the tundra already paired, and arriving birds distribute themselves over the southern parts of the breeding range according to available snow-free habitats. Site fidelity is low between years and numbers at single sites fluctuate widely. New arrivals tend to settle close to early pairs. After egg-laying, as snowmelt and predation on clutches develop, most males, and females with failed nests move farther north in Taimyr for a new breeding attempt where the environment corresponds to an earlier stage of spring in the south. These birds form a second breeding wave. Males leave the breeding grounds by early July; if breeding fails females leave soon afterwards; in successful breeding years they depart in late July–early August. Most juveniles leave in early August. The paper discusses the key features of the Curlew Sandpiper breeding system, and presents a model to explain the peculiar breeding population structure and breeding schedule of this species, for example the bimodal pattern of clutch completion.