Prof. Wells W. Cooke (1912) made the statement that this species has two breeding areas "separated by nearly 1,500 miles of Arctic coast, from Point Barrow to the Boothia Peninsula," where "there seems to be no certain record of the occurrence of the red-backed sandpiper." This is far from true, for it is well known to breed there and eggs have been taken at many places along the Arctic coast.
Eggs.—Herbert W. Brandt (Mss.) describes his series of 120 eggs of this sandpiper very well, as follows:
The four eggs of the red-backed sandpiper, which is their complement, are very handsome and show more variation than the eggs of most of the other shore birds breeding in the Hooper Bay region. In shape they are subpyriform to ovate pyriform and rest amid the leafy nest lining with the small ends together often so placed that the sitting bird during incubation touches only the larger ends. The shell is not as strong as many shore-bird eggs of the same size but they are not fragile by any means and they have considerable luster. As was true of many of the limicoline eggs found along that Bering Sea coast, there were two general types of ground color—the one, the greenish, that predominated by a ratio of about 15 to 1—and the other was the brownish type. The ground color ranges from "pale glaucous green"—that is the most common type—to "glaucous green," while the brownish-tinged eggs shade from "olive buff" to "dark olive buff." The surface markings are conspicuous and vary greatly, for on some types the spots are small and well scattered over the eggs; on others they are large, irregular, and bold; while on still other specimens they are confluent on the larger end and form a blotch that completely decorates that part of the egg.
These spots are irregular in shape, but are inclined to be elongated with their axis twisting to the right, so that when a series of eggs is viewed looking toward the larger end, the spots produce a clockwise spiral. Some of these spots are more twisted than others, but on a few eggs there are no spiral tendencies at all. The surface spots are quite variable in color, dependent largely on the thickness of the pigment deposited, for where the latter is thin the true color is observable, but when the decoration is liberal, the blot becomes opaque and the color is lost. These spots range from "auburn" and "raw umber" to "chestnut brown" and "blackish brown." The underlying spots are well hidden by the boldness of the surface markings and inclined to be small and regular and are often more or less numerous. Their shades are delicate, ranging from "pallid gray" to "mouse gray." An occasional egg exhibits scattering insignificant additional markings of deep "blackish brown."
The measurements of 145 eggs average 36.3 by 25.3 millimeters; the eggs showing the four extremes measure 40.1 by 25.9, 39.2 by 26.5, 34 by 25, and 34.5 by 23.5 millimeters.
Young.—Both sexes incubate and are rather close sitters, as well as devoted and bold in the defense of their young. The period of incubation is probably the same as for the European dunlin, 22 days. John Murdoch (1885) says:
Both parents share in the work of incubation, though we happened to obtain more males than females with the eggs. The young are pretty generally hatched by the first week in July, and both adults and young keep pretty well out of sight till the 1st of August, when they begin to show about the lagoons and occasionally about the beach, many of the young birds still downy about the head. The autumn flight of young birds appears about the middle of August, associating with the young A. maculata and M. griseus scolopaceus, in good-sized flocks, particularly about the pools on the high tundra below Cape Smythe. They continue plenty in these localities, sometimes appearing along the beach, for about a week, when the greater part of them depart, leaving only a few stragglers that stay till the first few days of September.
Plumages.—The downy young red-backed is much paler and more buffy than that of the least sandpiper. The crown, back, wings, and thighs are variegated with brownish black, "ochraceous tawny" and "hazel," except at the base of the down on the back, there is no rich, deep brown; the above parts are quite thickly sprinkled, especially on the back, with minute, round spots, terminal tufts of pale buff; a distinct stripe of these nearly encircles the posterior half of the crown. The black of the crown extends nearly to the bill and there is a black loral stripe; the rest of the head and a band across the lower throat are "warm buff." The rest of the under parts are white. The nape is a grizzly mixture of dull buff and dusky.
The juvenal plumage, as seen in Alaska in August, is strikingly handsome and quite distinctive. The feathers of the crown are dusky, edged with "ochraceous tawny"; the sides of the head and nape are "drab-gray," streaked with dusky; the feathers of the back are black, broadly edged with three colors in different areas, "ochraceous tawny," "hazel," and buffy white; the scapulars are black, edged with "light ochraceous buff"; the wing coverts are gray, tipped with pale buff; the rump and upper tail coverts are "hair brown" to "drab"; the breast is tinged with grayish and pale buff and streaked with dusky; the throat and rest of the under parts are white, conspicuously and more or less heavily spotted with dusky on the sides of the belly. This beautiful plumage is worn for only a short time and is molted before the birds leave their northern breeding grounds. The postjuvenal molt begins in August and is generally finished before October; it involves nearly all of the body plumage, nearly all of the scapulars, and some of the tertials, but not the rump, upper tail coverts, or flight feathers.