Adults have a complete postnuptial molt between August and November and a partial prenuptial molt from January to May; this latter involves most of the body plumage, but not all of the scapulars, back, rump, or upper tail coverts.
_Food._--The favorite feeding places of purple sandpipers are the wave-washed rocky shores of islands or promontories along the seashore, with a decided preference for islands. Here, where the rocks are fringed with rockweed, waving in the restless waves, or covered with barnacles and various slimy products of the sea, these sure-footed little birds are quite at home on the slippery rocks, as they glean abundant food at the water's edge and skillfully avoid being washed away. Yarrell (1871) says that--
it may be seen busily employed turning over stones and searching among seaweed for the smaller shrimps and sandhoppers which are to be found there, and it also feeds on young crabs, marine insects, and the soft bodies of animals inhabiting small shells.
Witherby's Handbook (1920) gives its food as--
varied, including insects: coleoptera (_Otiorhynchus_), diptera (larvae of _Chironomus_), also spiders, _Thysanura_ (or _Collembola_), annelida and crustacea (_Amphipoda_, _Isopoda_, _Orchestia_, _Idotea_, _Gammarus_, and _Podocerus_) as well as mollusca (_Mytilus_, _Littorina_, _Purpura_, etc.). Vegetable matter is also eaten including algae, grasses, moss, buds, and leaves of phanerogams and remains of cryptogams. Seeds of _Cochlearia_ have been identified and small fish (_Gobius_) nearly 1 inch long, as well as ova of lumpsucker.
_Behavior._--The flight of the purple sandpiper suggests at times that of the spotted sandpiper, for when disturbed singly along the shore it is apt to fly out over the water with rapid downward wing strokes and, describing a large semicircle, return to the shore some distance ahead. When flying in a flock the birds are often closely bunched, the whole flock wheeling and turning in unison, showing alternately their dark bodies and their white bellies, in true sandpiper fashion. As a rule they do not make very long flights or fly very high. Their migrations are short and deliberate. They are rather sedentary birds and can generally be found in certain favorite localities all winter and year after year. But, as they show a decided preference for the outer sides of surf-swept ledges, they are not often seen from the land. They can swim almost as well as phalaropes and in calm weather they will often alight on half submerged seaweed or on the surface of the water. Dr. Charles W. Townsend (1905), who watched a flock on an island off Cape Ann, describes their actions as follows: