Migratory shorebirds that live in the tropics prior to embarking on long (>5000 km) flights may face heat-load problems. The behaviour of a large sandpiper, the Great Knot (Calidris tenuirostris), was studied in Roebuck Bay, north-west Australia, from February to April 2000. We determined the incidence of heat-reduction behaviour in foraging and roosting birds in relation to breeding-plumage score (an index of migratory preparation) and microclimate variables. Heat-reduction behaviour (primarily raising the back feathers) was significantly related to breeding-plumage score and solar radiation. Raising back feathers may reduce the external heat load for a bird, or increase convective or cutaneous evaporative cooling. The results suggest that managing heat loads in tropical-wintering waders may become more difficult close to departure on migration.