As the spring shorebird migration is underway I wanted to review the excellent article by Cameron Cox from American Birding in the July/August 2008 edition. This article reviews the classic North American small sandpipers we call peeps, and rather than focusing on subtle plumage differences and bill length differences, drills down on structural differences to separate these first into 3 groups, and then down to species. He separates N. A. peeps first into 3 groups:

Least Sandpiper:
Behavior: The smallest sandpiper in the world. Rarely form large tightly packed flocks. Crouch to feed, breast nearly touching the ground, often on periphery of flocks, seem nervous.
Structure: Appear dinky, round headed, short finely tapered bill, large round eyes seem prominent, very short primary and wing extension, not at all beyond the tertials. Leg bones and joints finer than other peeps.
Flight: Short rounded wings, noticeably bent back at the wrist, narrower based wings than other peeps in a flock.
Voice: Flignt call a shrill rising Trreee

Standard Peeps (Western Sandpiper & Semi-palmated Sandpiper):
Behavior: Often form huge flocks on mudflats. Feed by dropping their heads while keeping their breasts well above the ground (vs Least), tarsi angled back only slightly while feeding (vs. crouch-kneed least), feed well in front of their feet, and seem more focused than Least, looking around less often usually.

Structure: Legs more substantial than least, “knees
knobby,” both species have slight webbing between toes.

Semi-palmated Sandpiper (SESP): molt body in migration, but rarely show full basic plumage in U.S. or Canada. Never molt wing feathers before wintering grounds. Late fall SESP first year birds retain brown juvenile plumage vs. WESA already in gray first-basic (formative) plumage.
Behavior: Often more aggressive that WESP to nearby birds.
Structure: “Compact” both breast and belly “robust” leaving smoothly rounded underparts and head appearing small relative to WESP. May look “bullnecked” relative to “rangier” WESP. Legs near center, making SESP look like weight is evenly distributed. Bill straight with blunt, even swollen tip. (quite variable)

Western Sandpiper WESP): Only WESP and LESP likely peeps in winter in N. A. WESP molt much earlier than SESP. Some attain full basic plumage by August. Primary and secondary wing molt can occur in migration, so can be seen in N. A. Any first cycle standard peep with extensive pre-formative molt is likely a WESP.
Structure: Lankier and longer-legged that SESP. Head appears slightly too large for body size. More weight appears in front of the legs. Look “heavy-chested” Bill usually slightly droopy.
Voice: quite different from SPSP- a low rolling chrrk vs WESP a low rolling chrrk vs WESP a high sharp shik.

Long-winged Peeps: (Baird’s Sandpiper and White-rumped Sandpiper): Built for long migration between high arctic breeding and far southern SA wintergrounds. Long wing projection, look long and attenuated. Both have more body projecting behind the legs than standard peeps. Wing tips so long that they frequently cross.

Baird’s Sandpiper: Much more common than WRSP in the west. Earlier migrant than WRSP in both spring and fall. May be seen at high elevation in mountains. May move slowly, like a shuffling gait. Tends to bob head back and forth while walking.

Structure: Small headed, bill fine the entire length. Whole body has a distinct flattened appearance, seen head-on like an egg on its side. Short legs and flattened appearance can make it look like it is “hugging the ground”

White-rumped Sandpiper: Late spring migrant thru middle N. A. but fall mostly off shore in Atlantic, though regular on East Coast and central N. A. in fall. Relatively late in fall also, juveniles can be seen into Nov. Forward tilt quite dramatic compared to other peeps.

Structure: Heavier billed, and overall bulkier and larger headed than BASP.

Flight: P-10 longest in long-winged peeps vs P-9 in rest of peeps. Hard to see in flight can be seen in photos. Long hands that gradually taper in both long-winged peeps.
WRSP especially can do long glides.

Voice: WRSP has a high-pitched explosive siit and sharper and more piercing than WESP, and BASP has soft dry crrep more like Pectoral sandpiper but softer. Check out the photos in the article linked above to look at these structural differences in the peeps. Certainly plumage differences help a lot too, and here are some very basic ones to remember: Least Sandpiper: Yellow legs when seen diagnostic, dark breasted, dark above overall in adults, juvenile bright rufous overall vs more localized to scapular and cap in WESP.

Standard Peeps: Adult SESP more gray brown overall and WESP has more distinct rufous scapulars. Juvenile SESP with dark central feathers vs dark central anchors on upperparts. Juvenile WESP “cookie-cutter similar, bright scapulars, vs juvenile SESP darker above with minimal rufous scapulars. Long-winged Peeps: juv. Bairds with pale buffy breast band, scaly above. Juv. WRSP dirtier breast, brightly patterned above and with large white rump in all plumages in flight. (all other peeps stripe-tailed)” target=”_blank”>high sharp shik.

Long-winged Peeps: (Baird’s Sandpiper and White-rumped Sandpiper):

Built for long migration between high arctic breeding and far southern SA wintergrounds. Long wing projection, look long and attenuated. Both have more body projecting behind the legs than standard peeps. Wing tips so long that they frequently cross.

Baird’s Sandpiper: Much more common than WRSP in the west. Earlier migrant than WRSP in both spring and fall. May be seen at high elevation in mountains. May move slowly, like a shuffling gait. Can bob head back and forth while walking.
Structure: Small headed, bill fine the entire length. Whole body has a distinct flattened appearance, seen head-on like an egg on its side. Short legs and flattened appearance can make it look like it is “hugging the ground”

White-rumped Sandpiper: Late spring migrant thru middle N. A. but fall mostly off shore in Atlantic, though regular on East Coast and central N. A. in fall. Relatively late in fall also, juveniles can be seen into Nov. Forward tilt quite dramatic compared to other peeps.
Structure: Heavier billed, and overall bulkier and larger headed than BASP.
Flight: P-10 longest in long-winged peeps vs P-9 in rest of peeps. Hard to see in flight can be seen in photos. Long hands that gradually taper in both long-winged peeps. WRSP especially can do long glides.
Voice: WRSP has a high-pitched explosive siit and sharper and more piercing than WESP, and BASP has soft dry crrep more like Pectoral sandpiper but softer.

Check out the photos in the article linked above to look at these structural differences in the peeps.

Certainly plumage differences help a lot too, and here are some very basic ones to remember:

Least Sandpiper: Yellow legs when seen diagnostic, dark breasted, dark above overall in adults, juvenile bright rufous overall vs more localized to scapular and cap in WESP.

Standard Peeps: Adult SESP more gray brown overall and WESP has more distinct rufous scapulars. Juvenile SESP with dark central feathers vs dark central anchors on upperparts. Juvenile WESP “cookie-cutter similar, bright scapulars, vs juvenile SESP darker above with minimal rufous scapulars.

Long-winged Peeps: juv. Bairds with pale buffy breast band, scaly above. Juv. WRSP dirtier breast, brightly patterned above and with large white rump in all plumages in flight. (all other peeps stripe-tailed)