Plumages.—The downy young white rumped is much like the downy young of other tundra nesting species of sandpipers. From the stilt sandpiper it can be distinguished by its much shorter legs and shorter and slenderer bill, from the Baird by its more buffy face and breast, these parts being pure white in bairdi, and from the least by paler and duller browns in the upper parts and by white, instead of buffy terminal tuft spots. The crown, back, rump, wings, and thighs are variegated or marbled with "Sanford's brown," or "tawny," and black, dotted, except on the front half of the crown, which is mainly bright brown, with whitish terminal tufts. The forehead, a broad superciliary stripe, the sides of the head, throat, and breast are pale buff or buffy white; the remaining under parts are grayish white. A median frontal stripe of black terminates in "tawny" toward the bill; there are extensive black areas on either side of the crown and on the occiput. The nape is grizzly, buff, gray and dusky.
In juvenal plumage the crown is sepia with "tawny" edgings; the back, rump, tertials, and scapulars are sepia, with "tawny" edges, and some of the feathers of the mantle and scapulars are also white tipped; the under parts are white, but the breast is suffused with light buff and narrowly streaked with dusky; the median and lesser wing coverts are broadly edged with light buff or whitish.
The postjuvenal molt of the body plumage usually occurs in September and October, mainly in the latter month, but sometimes not until November. The upper body plumage is not all molted, so that first winter birds can be distinguished by tawny or buffy edged feathers in the mantle and by the juvenal wing coverts. The next partial prenuptial molt apparently removes all traces of immaturity.
Adults have a prenuptial molt, beginning in March, of the under-body plumage, most of the upper-body plumage, sometimes the tail, and some of the wing coverts. But this is almost immediately preceded by the delayed molt of the remiges in January and February, so that it seems to be a nearly complete prenuptial molt, which is barely finished before the birds start on their long northward migration. The postnuptial molt of adults, beginning in August and often lasting into October, involves only the body plumage, the tail, and some scapulars, tertials, and wing coverts. The gray winter plumage, so different from the brightly colored spring plumage, is seldom seen in its completeness before the birds go south.
Food.—Very little seems to have been published on the food of the white-rumped sandpiper, but W. L. McAtee (1911) gives it credit for eating some injurious insects and worms, such as grasshoppers, the clover-root curculio, which is injurious to clover, and marine worms (Nereis), which prey on oysters.
Stuart T. Danforth (1925) says that four collected in Porto Rico—
had eaten 77.7 per cent of animal food and 22.3 per cent of vegetable matter. Fifty per cent of the animal matter consisted of bloodworms, 25 per cent of Planorbis snails, and 5 per cent of Corixa reticulata. The vegetable matter consisted entirely of seeds, of which those of Compositae formed 33.3 per cent, Sesban emerus 30 per cent, and Persicaria portoricensis 36.7 per cent. In addition to food, the stomachs contained mineral matter (coarse red sand) forming 32.5 per cent of the stomach contents.
Behavior.—Lucien M. Turner, who has had abundant opportunity to observe this species in Ungava, writes in his notes:
The flight of these birds is remarkably firm and swift, generally in an undulatory manner and swerving to the right or left often with the body inclined to one side, the wing nearly perpendicular, alternately presenting the upper and lower surface of the body. Just before alighting the wings are raised until the tips nearly touch over the back and for a moment held outstretched and then slowly folded. The bird is quite active while searching for food and seldom remains more than a few seconds in a spot, where it constantly picks here and there for the minute organisms which form its food. During this time all is activity and quite in contrast to the interval while the tide is high and the bird is on the high land resting and digesting its food. Here it is more sleepy and less easily disturbed. The eyesight of these sandpipers is certainly very acute, as they are able to detect the presence of a person at a long distance and give a twittering, snipey note, otherwise regardless of approaching danger. In all their doings the utmost harmony seems to prevail. The only object of their lives seems to be to gather food. No sooner does the water begin to ebb than a few of these birds will swish over some point of land with merry twittering, eagerly scanning the bank for the least appearance of mud now being exposed. By the time the tide has half ebbed myriads of these birds are sweeping back and forth along the river. As the water shallows over well known bars, and scarcely has the water shoaled enough to permit the birds to alight without swimming, than as many sandpipers as can collect on the place eagerly alight and begin probing the ooze for food. The lowering water is followed by the thronging birds to the last inch.
The flowing tide begins and the birds retreat carefully seizing every object of food that the rising water brings to the surface. Often they are so eager in their search that many birds are crowded into the deeper water and save themselves only by flight. This or that place is quickly covered over by the water and again the birds collect into larger flocks which now sway to the right or left, alternately, exposing their silvery white underparts which gleam in the sunshine like a stream of silver. The gray or brown of their backs relieving the color as the long stream of birds pass by. They partially halt their flight and become a compact flock, whereupon they separate into smaller flocks which, as the water gradually rises and covers their feeding grounds, now betake themselves to the higher lands of the banks above. Here, around the pools on the highlands, or among the grassy margins of the lakes, they collect to wash themselves and digest the food they have obtained from the salt-water mud.