Of our two most common small Calidris sandpipers in eastern North America, Semipalmated (SESA) is always paler and grayer than Least Sandpiper (LESA). Be sure to compare birds of the same age and in the same stage of molt; see Fig. 16.

Leg color ranges from dark gray to dark green to black; see Fig. 1.

Beginning in spring, winter (basic) plumage is slowly replaced by spring (alternate) plumage; see Fig. 3. The winter (basic) feathers on this bird have pale, very narrow margins because they are worn. Fresh spring (alternate) feathers have thick blackish centers and wide gray margins. On a breeding-aged bird, two years or older, in full spring (alternate) plumage, the crown is streaked with alternating dark and rusty feathers. The breast has coarse streaks that thin out toward the middle of the breast. Some faint streaks or flecks continue on the flanks; see Fig. 4.

As spring turns into summer, SESA alternate plumage begins to look different because the feathers become worn. Beginning in July, summer (alternate) plumaged birds start to molt to winter (basic) plumage and display a salt-and-pepper appearance as the worn, dark, older feathers are replaced with new paler feathers with wide margins and dark central shafts; see Fig. 5, Fig. 6, Fig. 7, Fig. 8.

SESA almost never molts its flight feathers during migration, waiting instead until it reaches its wintering areas.

Newly fledged young birds arrive from the Arctic in late summer, displaying the characteristic fresh, evenly patterned, scaly look of young shorebirds; see Fig. 10.

In Fig. 9, a molting summer (alternate) SESA stands next to a juvenile. Note the cold gray of the upperparts on both. On the juvenile bird (right), each gray lower back (mantle) feather has a sharply defined dark shaft streak surrounded by paler brown. All the juvenile mantle feathers have broad pale margins, giving the mantle an evenly patterned or scaly look. By contrast, the bird in molting alternate plumage (left) has an uneven pattern of dark and pale feathers, giving it a salt-and-pepper look.

Birds in the patterned, fresh juvenile plumage can look buffy all over and may display some tawny or rufous feather edges on the back (mantle); see Fig. 10 and Fig. 12.

Most juveniles, however, are grayish-brown; see Fig. 11, Fig. 13.

Fig. 14 illustrates a juvenile bird partially molted to winter (basic) in mid-fall.

In many juvenile SESA (and sometimes in birds of other plumages), pale mantle and lower scapular feather margins create two parallel Vs on the dorsal surface of the bird. These are easily visible in Fig. 10 and Fig. 12, not as conspicuous in Fig. 11.

Female Semipalmated Sandpipers have longer bills than males.

The bill, which is always black, is straight in the shortest-billed individuals, but often droops slightly at the tip in longer-billed birds.

When examined head-on, the bill has a slight bulb at the tip.

Semipalmated Sandpipers nesting in northern Ungava Peninsula and Baffin Island, Canada, are on average slightly larger and have longer bills than more western birds. These eastern-breeding birds are usually the last birds to pass through our area in spring migration and the first ones to return after nesting. Because females leave the breeding grounds shortly after the eggs hatch, before the males leave, we see large numbers of returning long-billed adult females in late July and early August. The predominance of long-billed SESA individuals in the early southbound adult migrant flocks on New England shores is striking; see Fig. 6.

In Fig. 4, the medium-long, very straight bill could indicate a female SESA.

In Fig. 3, Fig. 5, Fig. 7, Fig. 10, if you reverse the bill, it reaches just behind the eye. These are typical short-billed SESA males.

The birds in Fig. 11 and Fig. 13, each with a very long bill, appeared in mid-September. They were likely juvenile Semipalmated Sandpiper females from the eastern long-billed population.

The bird in Fig. 8 has a medium-long bill and is in heavy molt, slightly later in September. It, too, is possibly a SESA female.

When these birds walk in sand or a hard surface, it may be possible to see the partial palmations (webbing) between the toes; see Fig. 10.

When the wings are folded, SESA wing tips barely reach the tip of the tail or fall short of it; see Fig. 5.

In flight, SESA displays a narrow, white wing stripe and dark center to the tail; see Fig. 15.

Semipalmated and Least Sandpipers (SESA, LESA) are close in size and shape. For a comparison image, see Fig. 16. In all plumages and ages, note the following:

• Overall color: colder, grayer tones for SESA; warmer, redder tones for LESA. In particular, juveniles are most distinctive.
• Leg color: dark gray or dark green for SESA; pale yellow or yellow-green for LESA.
• Compared to LESA, breast and upper neck markings on SESA are not as pronounced; there is often a pale, unmarked stripe down the middle of the breast. This feature is extremely variable and should always be used in conjunction with other features to determine the species.
• SESA has partially webbed (semipalmated) toes, but the webbing is hard to see in the field. LESA’s toes are separated.
• Male SESA bill is stubby and blunt. Female SESA bill is longer and tapered but usually blunter than LESA’s fine-tipped and tapered bill.
• SESA posture is upright; LESA posture is hunched.
• SESA sometimes emits a low chatter but is often silent when flushed. LESA often calls a high-pitched jeer-jeer or kreet-kreet when flushed.

Semipalmated and Western Sandpipers (SESA, WESA) are close in size and shape. In the spring, the rare WESA individual in New England, with its distinctive alternate plumage, is relatively easy to pick out among the hordes of SESA migrants. In the fall, more WESA individuals arrive on the New England coast before heading south. Due to variations and overlap in bill length and variations in feather wear and molt, it is not always possible to separate WESA from SESA in the field, especially juveniles. It is safest to count as WESA only individuals with long bills and some rufous in the upper scapulars. The situation is more complicated from New Jersey south because many more WESA mix with the migrant flocks of SESA during fall migration at more southerly latitudes along the east coast of North America.

Note the following:

• The rufous color in spring and summer (alternate) plumaged WESA upperparts is concentrated in the crown, cheeks, and scapulars, contrasting with the gray of the rest of the upperparts. SESA in alternate plumage may show some rufous on the crown and behind the eye. Or the rusty color may be scattered over other parts of the upper plumage, but it is not concentrated in the scapulars; see Fig. 4.
• In alternate plumage, SESA has a streaky breast band, and some streaks and flecks extend to the flanks; see Fig. 4. WESA in alternate plumage often has arrow-shaped marks all along the flanks nearly to the tail; see WESA Fig. 6.
• SESA retains juvenile feathers later into the fall than juvenile WESA. This is because the molt to winter (basic) plumage does not complete until the birds are near or on their winter range. Few SESA winter in the southern US, so the juveniles retain most of their juvenile plumage until they reach the Caribbean, Mexican, Central American or South American wintering areas in November or December. Many WESA winter in the southern US, so their molt to winter plumage occurs earlier, often by September.
• Western Sandpipers (WESA) molt their flight feathers during migration; Semipalmated Sandpipers (SESA) almost never do. In a flying flock, birds with missing or half-grown flight feathers are most likely WESA.
• Female WESA have very long bills and are usually unmistakable in the field; see Fig. 18. Fig. 17 juxtaposes male and female Semipalmated Sandpipers (SESA) with a long-billed female WESA.
• The longest-billed (female) WESA and shortest-billed (male) SESA are easy to pick out in a flock. Others, not so easy. In Fig. 13, the lack of rufous on the scapular margins and the relatively straight bill support the identification of SESA as opposed to WESA.
• WESA and SESA both have partial palmations (webbing) between the toes.

White-rumped Sandpiper (WRSA) is noticeably larger than Semipalmated Sandpiper (SESA) and looks longer due to its long wings. WRSA’s white rump is usually invisible when the birds are feeding or resting. You can often pick out the white-rump when WRSA is in flight.

Note the following:

• The folded wing tips on WRSA almost always project beyond the tip of the tail, unlike SESA; see Fig. 19.
• The streaked gray head, nape, shoulders, neck, and upper breast on WRSA create the impression of a cowl and an overall grayer bird than SESA.
• Faint streaks often line the flanks of WRSA all the way to the undertail.
• Male SESA bill is short and stubby. WRSA in general has a longer, slightly curved and more tapered bill
• Many White-rumps (not all) have a pale brown or reddish base to the lower mandible.
• Most WRSA seen in New England until late September are molting summer (alternate) plumage birds. Juvenile WRSA usually appear in October; juvenile SESA are here by August and often depart by mid-October.

Dunlin (DUNL) is much larger than Semipalmated Sandpiper (SESA), and has a much longer bill.

Note the following:

• In spring and summer (alternate) plumage, DUNL has bright rusty upperparts and a black patch on the belly; SESA is grayish overall with some brown or rufous highlights on the crown and mantle and a pale belly.
• During southbound migration, few DUNL arrive at New England sites before October. Most SESA are ready to depart or have already departed by then.
• In winter (basic) plumage, DUNL is smudged with gray on the upper breast and neck with some fine gray streaks on the lower breast and belly. SESA in late fall is clean breasted with few markings in the center of the neck and upper breast; see Fig. 20.

Sanderling (SAND) is much larger than Semipalmated Sandpiper (SESA), with thicker legs and a thicker bill.

Note the following:

• In spring (alternate) plumage, SAND has highly speckled cinnamon-rufous and black upperparts, head, and neck. SESA spring (alternate) mantle plumage consists primarily of brownish-gray feathers with some rufous highlights on the crown.
• The juvenile plumage of both SAND and SESA is highly patterned. SESA displays a scaly brown-and-white mantle pattern; SAND’s mantle is a boldly checkered black-and-white; see Fig. 21.
• SAND winter (basic) mantle plumage is the palest of grays, compared to the brownish gray of SESA.