Nest scrape design and clutch heat loss in Pectoral Sandpipers (Calidris melanotos)

Summary 1. The reasons why birds construct nest scrapes, and the extent to which scrape designs reflect functional optima, are poorly understood. Working on Pectoral Sandpipers (Calidris melanotos, Vieillot), we investigated whether scrapes function to insulate clutches and are efficiently designed to reduce heat loss rates. 2. Excavating a scrape and using lining material reduced the rate at which an object positioned within a scrape lost heat by 9% and 25%, respectively, suggesting that lined scrapes insulate clutches. 3. The rate of heat loss from an object within a scrape increased with scrape depth and decreased non-linearly with lining depth. The extent to which wind increased forced convective heat loss decreased with scrape depth. 4. On average, Pectoral Sandpipers used the minimum lining depth that approximately minimized heat loss through the lining. Mean scrape depth approximately minimized convective cooling in windy conditions while minimizing heat loss to the ground. Pectoral Sandpiper scrapes therefore efficiently reduced heat loss given conflicting environmental thermal pressures. 5. Available lining materials differed in insulative quality when both damp and dry. Pectoral Sandpipers used lining materials that insulated relatively well when damp more than expected given random collection of locally available materials. Linings therefore insulated efficiently given the damp nesting environment.