Cox's sandpiper has never been observed in breeding plumage. All mature specimens that have been observed have been in non-breeding plumage, although some have started to acquire a few breeding-plumage feathers. In non-breeding plumage, birds are brown-grey above and white below, with a brown-grey breast-band and no flank-streaks. When hints of a breeding plumage are acquired, a rusty tinge develops on the breast and ear-coverts, some flank streaks appear, and on the upper parts the non-breeding-plumaged feathers are replaced by feathers with black centres, grey tips and buff or pale chestnut fringes.