So far as known, the food of both old and young consists of beetles and flies while the birds remain on the highlands; when they move to the ponds and seashores they eat copepods, amphipods, etc. As soon as the young birds are well able to fly they resort to the tide pools and small ponds near the sea. Later the older birds join them and the flocks increase in size to several hundred in favorable places. This takes place in August and September in such localities as the Salt Lagoon of St. Paul Island.
Plumages.—The color pattern of the downy young Pribilof sandpiper is similar to that of the Aleutian, but the colors are different, much duller. The bright browns and buffs of the upper parts are replaced by "burnt umber," "snuff brown," "clay color," and "cinnamon buff," and the black markings are largely replaced by dark browns; the black patch in the center of the back is about as in the Aleutian. The under parts are less pure white, always suffused with pale buff on the throat and flanks and sometimes largely so on the breast also.
Mr. Palmer's (1899) studies of the molts and plumages indicate that they are similar to those of the Aleutian sandpiper; he writes:
The downy young are beautiful little things, silvery white beneath, bright, rich ocherous above, variegated with black and dots of white. The general color above lacks the grayness of the similar age of maritimus. The white dots are interesting under the microscope. They are composed of a bunch of highly specialized down, in which the radii near the tip are crowded and colorless. As they grow older the first feathers appear on the sides of the breast, on the back and scapulars; then the primaries and larger wing coverts appear. The feathering continues until the breast and under parts are covered, when the tail appears. At this time there are no feathers on the rump or on the head or neck. In the next stage feathers have appeared on the occiput and on the auriculars and are also extending up the neck. At the same time the tips of the back feathers have become somewhat worn, so that the colored margins are narrower and the black more prominent. The wing coverts are also to some extent worn on their tips. When the bill is an inch long the down has nearly all disappeared, and when it has entirely gone the birds appear in small flocks on the beaches, the young generally keeping together. Then another change takes place, for the entire plumage now gives way to another, that in which the bird passes the winter. A few late July, immature birds show the beginning, for No. 118832, im. [male], July 29, has a few new feathers on the middle of the back and on the scapulars. They soon extend all over the back, so that specimens collected up to August 10 have many of the new whitish feathers on that region.
The contrast is striking between these feathers, the latest being of an almost even shade of pale plumbean with darker centers and generally with a narrow white margin. There are no specimens to show the complete change, but it is probable that these young birds remain on the islands until it is completed. By the middle of June the adults have fully changed to the breeding plumage, but on some specimens a few feathers of the previous winter's plumage persist much later. Thus on many specimens some alternate feathers of the scapulars and tertials are of the previous winter's well-worn plumage. In fact, few specimens are free from these old feathers. Soon after the middle of July the new plumage of the next winter begins to appear. At first a few feathers show about the breast, then on the scapulars, thence up the neck and over the head, so that by the 10th of August they have changed one-half. It would thus appear that before this species leaves the islands they assume entirely their new dress. And at this season, August 10, old and young flock together for the first time, and confine themselves to the sand beaches and surf margins about the islands for a few weeks, when they take flight by the 1st or 5th of September, and disappear until the opening of the new season.
The Pribilof sandpiper is much paler in the juvenal plumage and grayer in the winter plumage than the Aleutian.
Food.—Preble and McAtee (1923) report on the contents of 192 stomachs, as follows:
The articles of food composing more than 1 per cent of the total were: Mollusks, 32.63 per cent; crustaceans, 29.15 per cent; flies (Diptera), 23.49 per cent; beetles, 10.29 per cent; marine worms, 1.27 per cent; and vegetable matter, chiefly algae, 1.21 per cent. The vegetable matter, besides algae, included bits of moss and a few seeds of grass, lupine, violet, crowberry, and bottle brush.
Behavior.—Referring to the habits of Pribilof sandpipers, Mr. Palmer (1899) says: