In first winter plumage young birds are much like adults, but the ashy brown upper parts are usually somewhat paler, and they can always be recognized by the juvenal wing coverts and a few retained scapulars and tertials. A partial prenuptial molt, similar to that of the adult, produces a first nuptial plumage, in which young birds can be distinguished only by the retained juvenal wing coverts. In fresh plumage the black belly patch is veiled with white tips, which soon wear away and leave this area clear black.
The first postnuptial molt of young birds and the corresponding molt of adults produce adult winter plumages. The molt is complete and begins in July or even late in June; the wings are apparently molted first in July, and are entirely renewed before the birds start to migrate; the body molt begins in August and lasts through September; there are usually traces of the old nuptial plumage left when the birds arrive here on migration. The partial prenuptial molt of adults comes in April and May and involves the body plumage, but not all the scapulars or rump or wing coverts.
Food.—Red-backed sandpipers obtain their food on the ocean beaches at low tide, on sandy flats or on mud flats, often feeding in company with sanderlings, or with other small shore birds. Some writers have referred to them as nervous and active running about in a lively manner while feeding, but I have usually found them rather sluggish and inactive at such times, easily approached and unsuspicious. Their food consists of small mollusks, sand fleas, and other small crustaceans, amphipods, flies and other insects and their larvae, diving and other aquatic beetles, marine worms, and occasionally a few seeds of aquatic plants. They are apt to gather where fish cleanings and other offal are thrown out, to feed on the flies and other insects that abound there. Dr. Charles W. Townsend (1905) writes:
In feeding they frequently plunge the bill, slightly open to its base in the soft sand or mud, appear to work it about and when successful draw forth an amphipod or a worm. Several times on one occasion I saw one draw a worm to the water close at hand as if to wash it before swallowing it. On another occasion a couple of dunlins were so tame that it was possible to approach within 5 feet of them. They were diligently probing in the sandy mud, wading in water up to their bellies. At this depth it was necessary for them to immerse their heads entirely, and I could see them shut their eyes as they went under water. Whether the eyes were afterwards opened or not I am unable to say. When disturbed they flew but a short way, and if they happened to alight in water too deep for their legs, they swam readily, as do all shore birds. When disturbed the dunlin utters a short kuk. Their call note is distinctive, and resembles somewhat the word purre, by which name the European species is called. The note is plaintive and sometimes melodious, and recalls, without its harshness, the cry of the common tern.
Behavior.—The earlier writers refer to this as an active, restless bird. Audubon (1840) says:
There seems to be a kind of impatience in this bird that prevents it from remaining any length of time in the same place, and you may see it scarcely alighted on a sand bar, fly off without any apparent reason to another, where it settles, runs for a few moments, and again starts off on wing.
Giraud (1844) writes:
It is a restless active bird and gleans its food with great nimbleness, and seems to be fond of continually changing its position. Soon after alighting they collect together and make a short excursion over the water, again alighting a short distance from where they had previously taken wing. During their aerial excursions, when whirling about, they crowd so close together that many are killed at a single shot. On one of these occasions Mr. Brasher informs me that he killed 52 by discharging both barrels into a flock. This number is greater than I ever before heard of; but from 10 to 15 is not unusual.
Wilson (1832), writing when shore birds were abundant, says of this flocking habit: