Fall.—The beginning of the fall flight is apparently southeastward from its breeding grounds west of Hudson Bay and perhaps southward from Baffin Island, where it breeds. Mr. Turner's Ungava notes give a very good idea of this first step in the migration and a good impression of the great abundance of this species; he writes:
As I proceeded farther northward I did not observe a single one of these sandpipers until we came to anchor off the mouth of Georges River (July 31) where quite a number were seen on the pebbly beach, seeking their food among the rocks and shingle as the tide receded. At this date quite a number, in fact fully three-fourths, of those seen were birds of the year as was fully attested by traces of downy plumage yet among the feathers of the body and especially on the head and neck. The sizes of the flocks varied from three or four to nearly a dozen, doubtless consisting of a single brood or, in the case of the larger flocks, of two or more broods with their parents. Some of the younger members of the flocks had the wing quills not fully developed while others were considerably farther advanced. Such a variation of plumage both in age and coloration was exhibited that I presumed there must be two weeks difference in the ages of the different young.
By the 10th of August all the young are well able to fly and make protracted flights in search of food. By this date they assemble in flocks, amounting at times, to thousands of individuals, resorting to the mud flats left bare by the receding tide. The mouth of the Koksoak and the cove to the westward of it present excellent tracts of mud deposited in the little indentations. By the middle of September these birds begin to depart to the south. Many of them ascended the Koksoak and others doubtless followed the windings of the coast down the Atlantic. I have seen numerous flocks over a hundred miles from the mouth of the river as late as October 12th, and an occasional single bird as late as the 20th of that month. I have observed, at the mouth of the Koksoak River, flocks of these birds often numbering over a hundred individuals suddenly appear from high in the air. These I suspected to be birds coming from the regions to the northward of the strait for they always came from the sea.
Thence there is an overland flight to the Atlantic coast. Mr. Brewster (1925) says:
White-rumped sandpipers visit Lake Umbagog regularly and not infrequently, if rather sparingly, in autumn, appearing oftenest during the month of October. Those arriving early in the season are mostly adult birds which occur singly or two or three together; those coming later are of various ages and sometimes in flocks containing as many as eight or nine members each, but rarely, if ever, a greater number. Bonaparte's sandpipers are hardy birds. They may be seen at the lake when its bordering marshes are stiff with frost. Once (October 26, 1883) I found nine of them near the mouth of Cambridge River two days after the entire region had been covered with snow to a depth of 7 inches.
This bird is a regular, but never an abundant, migrant on the coast of Massachusetts in the fall. The vast numbers which Turner saw in Ungava must seek some other route; the species is never abundant in the interior in the fall and it seems to be rather rare on the Atlantic coast south of New England; the natural inference is that it migrates at sea from Maine or Nova Scotia directly to South America or the West Indies. It is abundant at times in Bermuda.
The adults begin to reach Cape Cod in August, but the main flight comes in September, consisting partially of young birds; most of the young birds come in October and some linger as late as November 10. While with us the white-rumped sandpiper frequents the wet meadows and marshes near the shore, as well as the sand flats, mud flats, and beaches, feeding at low tide singly or in small flocks and usually associated with pectoral, semipalmated, or least sandpipers. During high tides, while the flats are covered, this species may be seen on the high sandy beaches, mixed in with the vast flocks of small shorebirds, sleeping, or resting, or preening their plumage, while waiting for feeding time to come again; if the wind is blowing, all the birds are facing it; many are crouching on the sand and others are standing on one leg with the bill tucked under the scapulars. These flocks often contain hundreds and sometimes thousands of birds, mainly semipalmated sandpipers, semipalmated plover, and sanderlings. They are not all asleep, however, for if approached too closely, they all rise and whirl away in a vast shimmering cloud, flashing now white and now dark as they turn, and settle on the beach again at no great distance.
Winter.—August finds the white-rumped sandpiper migrating along the coast of Brazil and it has been known to reach Cape Horn as early as September 9. Doctor Wetmore (1926) writes:
The white-rumped sandpiper was the most abundant of the migrant shore birds in the regions visited in southern South America. The species was not recorded until September 6, 1920, when it appeared in abundance in southward migration on the lagoons at kilometer 80, west of Puerto Pinasco, Paraguay. The first flocks from which specimens were taken were adult females, and two taken on the date when they were first recorded had laid eggs a few weeks previous as was shown by the appearance of the ovaries. The southward migration came with a rush as the birds passed through the night as witnessed by their calls. The flight continued until September 21, when a dozen, the last seen here, were recorded. The birds circled about lagoons in small compact flocks or walked along on muddy shores, where they fed with head down, probing rapidly in the soft mud; anything edible encountered was seized and swallowed and the bird continued without delay in its search for more.
Farther south this species was encountered in abundance in its winter range on the pampa. Ten were recorded at Dolores, Buenos Aires, October 21, and from October 22 to November 15 the species was found in numbers on the coastal mud flats on the Bay of Samborombom. A few were seen at pools of water in the sand dunes below Cape San Antonio. Along the Rio Ajo white-rumped sandpipers were encountered in flocks of hundreds that came upstream to search the mud flats at low tide or were concentrated on bars at the mouth when the water was high. In early morning there was a steady flight of them passing to suitable feeding grounds. The birds flew swiftly, with soft notes, from 3 to 15 feet from the earth. In feeding they scattered out in little groups that covered the bare mud systematically. It was not unusual to record as many as 2,000 in a day. About two hundred were observed in the bay at Ingeniero White, the port of Bahia Blanca, on December 13, and at Carhue, Buenos Aires, from December 16 to 18, white-rumped sandpipers were noted in fair numbers on inundated ground back of the shore of Lake Epiquen or about fresh-water ponds on the pampa inland. None were found in Uruguay during February.