Food.—I have recorded the following found by me in the stomachs of this species taken on the New England coast; insects of various kinds, including beetles, small mollusks (Littorina), worms and crustaceans (Gammarus orchestia), bits of seaweed and sand. Dr. Alexander Wetmore (1916) records the contents of six stomachs from birds taken in Porto Rico in August; 99.16 per cent was animal matter, 0.84 per cent vegetable matter.
Beetles, bugs, fly pupae, and small mollusks form the bulk of the food. Small water scavenger beetles (Hydrophilidae) were found in four stomachs and amount to 27 per cent. Two ground beetles (Bembidium sp.) amount to 5 per cent and miscellaneous beetles to 3.34 per cent. One bird had eaten nothing but four back swimmers (Notonecta sp.), and these made 16.66 per cent. Fly pupae figure largely in two stomachs, forming 21.66 per cent of the total, and snails (Planorbis sp.) 13 per cent, while miscellaneous animal matter amounts to 12.50 per cent. The small quantity of vegetable matter present was rubbish. The numbers of Diptera eaten speak well for this sandpiper.
Preble and McAtee (1923) found in the stomach of a bird shot in the Pribilof Islands "remains of the beach beetle (Aegialites californicus), 10 per cent; fragments of small flies (Diptera), 85 per cent; and two seeds (not identified), 5 per cent."
Arthur H. Howell (1924) says, "Two stomachs of these birds from Alabama contained the remains of small mollusks, fly larvae and beetles. This species is known to feed on marine worms and mosquitoes."
Behavior.—Semipalmated sandpipers are fascinating birds to watch. When feeding on the beaches, they run along in a scattered flock just above the wave line, retreating rapidly as the wave advances, but sometimes being forced to flutter above it, all the time eagerly seeking for choice morsels. With head down, not held up as is the case with its companions the semipalmated plovers, it runs along dabbling here and there irregularly, and occasionally probing with its bill in the sand. These probings are not so deep nor so systematic as those of the sanderling, which makes a series of six to a dozen holes in succession throwing up the sand on either side. In its greediness the semipalmated sandpiper sometimes attempts to swallow too large a morsel for its small round mouth, which is much out of proportion to the stretch of the end of the bill, and many shakings of the head are needed to get a large morsel past the sticking point. I have seen one try several times to swallow a large beach flea (Talorchestia megalophthalma), and then fly off with it in its bill.
On a rocky shore I have seen them hunting for insects at high tide on the smooth rocks, and at low tide, running among the rocks covered with seaweed (Fucus vesiculsus) and on the floating weed, fluttering their wings from time to time to keep from sinking. Here they find plenty of food in the small mollusks and crustaceans, Littorina and Gammarus. On an August day on the coast of Maine I saw one searching about on floating rockweed several miles from land. Shore birds doubtless often rest in this way in their long journeys over sea.
In flight, semipalmated sandpipers in flocks, large and small, often move as one bird, twisting and turning with military precision, alternately displaying their light breasts and darker backs—flashing white and then almost disappearing. The method which enables shore birds, or, indeed, any flocking bird, to accomplish these evolutions is obscure. In the case of the semipalmated sandpiper these evolutions appear often to be made in silence, although it is of course possible that signals, not audible to the human observer, may be given. It has been suggested that telepathy or even that "a common soul" dominating the flock may be the interpretation, but both of these explanations are at present, at least, outside of scientific ken. I have noticed that birds who do not habitually execute evolutions, like English sparrows and the young of those that are skillful in this direction when adult, as for example, starlings, are much less proficient at this, and it seems to me possible that the whole thing may be accounted for by quickness of observation and of reaction, inherited and acquired.
Semipalmated sandpipers like other shore birds often stand on one leg and even hop along on it in feeding and they also sleep in this attitude. It is difficult to distinguish these from cripples, and one is easily deceived; the cripples seem as happy and tireless in feeding as the others.
William Brewster (1925) thus charmingly describes the habits of this bird in the wet and soft ground at Lake Umbagog: