Voice.—Following are Mr. Nichols's notes on this subject:
The common flight note of the stilt sandpiper is very like the single whistled whu of the lesser yellow-legs, but recognizably lower pitched and hoarser, at times with a quaver, whr-r-u, and varying down to a shorter, less loud whrug. An unloud, reedy sher has been heard from two birds when flushing.
Though with different feeding habits, stilt sandpiper, dowitcher, and lesser yellow-legs frequent the same grounds, associate very freely on the wing, and all three have a very similar flight note, though sufficiently different for identification. Perhaps the very lack of close relationship in these birds has facilitated convergence of their habits and calls, and it is not unreasonable to suppose that close association, even imitation, has played some part in bringing about the likeness of their voices. The greater yellow-legs differs more from the lesser, both in flight note and flight habits, than do these other two unrelated species.
Field marks.—I quote again from Mr. Nichols's notes on field characters, as follows:
On the wing the stilt sandpiper resembles the lesser yellow-legs closely. Its smaller size is scarcely appreciable, even in a flock of yellow-legs, the members of which will usually be at slightly varying distances from the observer. Adults have appreciably darker (barred) lower parts, and young birds, particularly, are greyer above than yellow-legs at the same season in this latitude. The somewhat shorter legs do not project so far beyond the tail, but the proportionately longer bill (with slight apparent drop at its tip) is the stilt sandpiper's best field mark. Its bill is proportionately longer even than that of the greater yellow-legs, with which this species is unlikely to be confused, varying as it does away from the lesser yellow-legs in an opposite direction, both as regards size and in other subtle characters. The head and neck of a yellow-legs are more "shapely," differing in this respect somewhat as a black duck differs from sea ducks.
On the ground the stilt sandpiper stands lower than a yellow-legs, having decidedly shorter legs, and correspondingly higher than our other shore birds of the same size. The color of its legs, dull olive green, is usually diagnostic. The legs are sometimes yellowish, and very rarely yellow, only one such having come under the writer's personal notice, a young bird in southward migration. The name "greenleg" is often used for it by Long Island baymen, who also suspect it of being a cross between yellow-legs and dowitcher. At sufficiently close range the margination of the feathers of the upper parts is quite unlike the spotting of the yellow-legs' plumage.
The broad white stripe over the eye is conspicuous in any plumage and the whitish tail shows in flight, as different from the whitish triangle on the rump and back of the dowitcher or the white rump of the yellow-legs. Most of these field marks, however, are too subtle for easy recognition, unless seen under favorable circumstances.
Prof. William Rowan has sent me the following notes:
Identification marks of the stilt are excellent and it is quite an easy bird to spot in almost any circumstances. It has a rump pattern all to itself and is therefore readily detected in flight. The end of the tail is darker than that of a yellow-legs, but the white of the rump end, instead of forming a straight line across the back, is horseshoe shaped. Although the turnstone and semipalmated plovers are reminiscent, they are quite distinct and not to be confused. When wading—the birds prefer to be belly deep—the carriage of the head makes the species unmistakable. The bill is always held and thrust beneath the surface perpendicularly. This necessitates a straight neck. In profile the feeding individual can be mistaken for no other sandpiper, is quite distinct from the yellow-legs, and can really only be confused with a phalarope. The Wilson phalarope habitually wades in this part of the world, swimming only occasionally, but its markings are distinctive. A flock of stilts is the most characteristic sight and the species can be identified at a great distance. The curious position of the head just referred to and the crowding of the individuals into each other make a quite unmistakable combination. They feed practically shoulder to shoulder, seldom scattering. The yellow-legs of a flock are always scattered, and the general aspect of the individuals is entirely different. Stilts never bob their heads after the manner of yellow-legs.
Fall.—The fall migration of adults begins very early, coming along with the dowitchers and first summer yellow-legs. I have an adult female in my collection, taken on July 5, 1885, on Monomoy Island, Massachusetts. The main flight of adults comes along during the latter half of July and first half of August, in this State, and the young birds come through in August and September; but this is a rare bird here, and the flight generally lasts for only a few days.
Mr. Nichols tells me that: