At Guamini, Buenos Aires, from March 3 to 8, white-rumped sandpipers were encountered in northward migration from a winter range in Patagonia. The species was fairly common on March 3 and increased greatly in abundance on the two days that followed. The northward journey was apparently as concerted as the movement that carried the birds southward, as on March 6 there was a noticeable decrease in their numbers, and by March 8, though the birds were still common, the bulk of individuals had passed. They arrived in flocks from the southward, often of several hundred individuals, that whirled in and circled back and forth along the lake shore to decoy to birds feeding on the strand or to rise again and continue swiftly northward. Those that paused kept up a busy search for food along the muddy beaches in or near shallow water, or in company with little parties of buff-breasted sandpipers on the drier alkaline flats back of the shore line. In early morning they were especially active and were in continual movement. Occasionally they worked out into comparatively deep water where in feeding it is necessary to immerse the head over the eyes nearly to the ear openings. When disturbed flocks rose with soft notes that resembled tseet tseet or tseup to circle to new feeding grounds on the lake shore.
The spring migration route is apparently northward along the Atlantic coast of South America and through the West Indies to the United States. Only a few migrate along our Atlantic coast, as the main flight is northward through the interior, during May and the first few days of June. Many reach their breeding grounds before the end of May.
Courtship.—Doctor Wetmore (1926) gives an attractive account of what seems to be a beginning of courtship in Argentina; he writes:
Occasional parties of males, animated by the approaching breeding season, broke into soft songs and called and twittered, often for several minutes, in a musical chorus in low tones that had so little carrying power that they merged in the strong wind, and it was some time before I succeeded in picking out the sweet individual songs tsep a tsep a tsep a or twee twee tee tee ty tee given as the head was bobbed rapidly up and down. Occasionally when the fall sunlight came warmly I sat in the mud and let little bands of white rumps work up around me until they were feeding and calling within a meter or so, eyeing me sharply for any cause of alarm. At such times their twittering choruses came sweetly and pleasantly, clearly audible above the lap of waves and the rush of the inevitable winds of the pampas. Between songs the search for food continued without cessation. At short intervals, activated by the warmth of the sun, they suddenly indulged in dozens of combats with their fellows, bloodless affrays, of bluff and retreat, where they lowered their heads and with open mouths ran at one another pugnaciously. The one attacked sidled quickly away or fluttered off for a short distance, save where two of equal temperament chanced to clash when first one and then the other threatened with raised wings in alternate advance and retreat until the fray was concluded to their mutual satisfaction. At such times the movements of these otherwise plain little birds were sprightly and vivacious to a degree. Their loquacity at this season was marked as it contrasted strikingly with their silence and quiet during the resting period of southern summer. Flocks frequently rose to perform intricate evolutions and then returned with a rush to sweep along the shore and join less ambitious comrades. As they passed the white rump flashed plainly, certain advertisement of the species. At times the chattering of these active flocks reminded me of the twittering of swallows.
J. Dewey Soper found this species quite common at various places on Baffin Island, and has sent me some very full notes on it, from which I quote as follows:
The species was encountered at Nettilling Lake, June 10, 1925, when a mated pair was flushed from a marshy upland near the Takuirbing River. The sexual organs in both were fully developed, the female being almost on the point of laying. By the 14th the species had become quite common. The males practised their vocal performance on the wing immediately upon arrival. They rise to a height of about 60 feet above the tundra and there they hover with rapidly beating wings giving utterance to their nuptial song in notes so weak that when a wind is blowing nothing may be heard of it even at comparatively close range. It is given in a very low tone and slow tempo; the notes are weak and inclined to be squeaky, with a weird dripping quality like the sound of water oozing and dropping in a small cavern. The species appears to flush silently.
Nesting.—Very little seems to be known and still less has been published about the nesting habits of the white-rumped sandpiper. In MacFarlane's notes I find brief records of seven nests found by his party on the barren grounds and Arctic coast of Canada, from 1862 to 1865. One nest was found on June 21, 1862, 50 miles east of Fort Anderson; the female and three eggs were taken; the nest consisted of a few leaves in a small hole in the ground near a small lake. Another found on June 29, 1863, on the barren grounds, is described as "a mere depression in the ground lined with a few decayed leaves"; the female and four eggs were taken. There is a set of four eggs in the Herbert Massey collection, taken with the parent bird by E. A. McIllhenny at Point Barrow, Alaska, on June 25, 1898; the nest is described as "a hollow in the moss on top of a ridge on the tundra, lined with dry grass and partly arched over." This is farther west than the species is supposed to breed, but there are several birds in the Philadelphia Academy of Sciences collected there in June, which are in full breeding plumage.
Mr. Soper found a number of nests near Nettilling Lake, Baffin Island, on which he has sent me the following notes: