コシジロウズラシギ 対 アメリカヒバリシギ

The most reliable way to separate White-rumped Sandpiper from Least Sandpiper is by spotting the bold white rump of White-rumped in flight—a feature Least Sandpiper never shows. On the ground, White-rumped is longer-winged and darker-legged, while Least is smaller, with yellowish legs and a shorter, finer bill. Juveniles of both can be confusing, but White-rumped juveniles show a distinctive white 'V' on the back and more contrast in the wing coverts, while Least juveniles are brighter above with rufous tones and white mantle stripes. Both species are small 'peeps', but White-rumped is noticeably larger and bulkier. Careful attention to leg color, wing projection, and rump pattern is key, especially in mixed flocks.

1. White rump in flight: Only White-rumped Sandpiper shows a bright white rump and uppertail coverts in flight. Least Sandpiper has a plain brown rump. This is the single most reliable field mark for all ages and seasons.
2. Leg color: White-rumped has black or very dark legs; Least Sandpiper has yellow-green legs. This is visible when birds are standing or walking, but beware mud-stained legs in Least.
3. Bill structure: White-rumped's bill is medium-length, thin, and dark; Least's bill is shorter, finer, and also dark. Bill length is best judged in direct comparison.
4. Wing projection: White-rumped has very long wings that extend well beyond the tail when perched; Least's wings are short and barely reach or just meet the tail tip. This is especially helpful when birds are resting or preening.
5. Size and jizz: White-rumped is noticeably larger, longer-bodied, and bulkier than Least, which is the smallest 'peep' and appears compact and short-necked.
6. Juvenile upperparts: Juvenile White-rumped shows a white 'V' on the back and more contrast in the wing coverts, with black bases and brown/white edges. Juvenile Least is brighter above with rufous tones and white mantle stripes, lacking the white 'V'.
7. Breast pattern: Both can show streaked breasts, but White-rumped often has finer, more extensive streaking onto the flanks; Least's streaking is usually limited to the upper breast.
8. Supercilium (eyebrow): Both have a pale supercilium, but White-rumped's is usually broader and whiter, especially in winter. Least's is thinner and less contrasting.
9. Crown and face: White-rumped has a brownish crown and face in breeding plumage, with a pale face; Least has a darker crown and a light line above the eye.
10. Leg length and stance: White-rumped appears longer-legged and stands more upright; Least is lower to the ground and often crouched.
11. Habitat context: If seen in southern South America in winter, it's almost certainly White-rumped; Least rarely winters that far south. In North America, both can occur together, so rely on structural and plumage marks.
12. Flock composition: White-rumped often associates with other medium-sized shorebirds; Least is more likely to be found in pure 'peep' flocks.
13. Winter plumage: Both become greyer above, but White-rumped remains darker and more contrasting, with a white supercilium and dark crown/eyeline. Least is plainer grey above.
14. Voice: Not diagnostic from the provided notes.
15. Pitfalls: Mud can obscure leg color in Least Sandpiper. Juvenile White-rumped can look bright above, but always check for the white rump in flight and longer wings at rest.

White-rumped Sandpiper is significantly larger and heavier than Least Sandpiper. White-rumped averages about 42.75 g in mass and measures 15–18 cm in length with a 36–38 cm wingspan [wikidata, wikipedia], while Least Sandpiper is the smallest North American shorebird at 19–30 g, 13–15 cm long, and a 27–28 cm wingspan [wikidata, wikipedia]. In the field, White-rumped looks longer-bodied and longer-winged, while Least appears compact and dainty.

Juvenile and immature plumages are often the source of confusion:

• Juvenile White-rumped Sandpiper: Shows black-based feathers on the back and wing coverts, with brown edges toward the scapulars and whiter edges toward the wing coverts. The breast is finely streaked, and a white 'V' is visible on the back. Underparts and supercilium are white, crown is brownish, and face is pale. In flight, the white rump is obvious.

• Juvenile Least Sandpiper: Brightly patterned above with rufous tones and white mantle stripes. The upperparts are more colorful and less streaked than White-rumped, lacking the white 'V' on the back. No white rump in flight.

• Adult breeding White-rumped: Dull grey-brown above with a pale supercilium, streaked breast and flanks, and white underparts. Crown and face have a brownish tinge, and wing coverts have grey edges. Long wings extend beyond the tail. In flight, the white rump is diagnostic.

• Adult breeding Least: Brown above with dark streaks, white below, light line above the eye, and dark crown. Legs are yellow-green, bill is short and thin. No white rump in flight.

• Non-breeding (winter) plumages: Both become greyer above, but White-rumped remains darker overall with a white supercilium and dark crown/eyeline. Least is simply grey above with less contrast. Leg color remains a key difference (dark in White-rumped, yellowish in Least).

Both species forage on mudflats and wet ground, picking at invertebrates. White-rumped Sandpiper often appears more deliberate and longer-legged, sometimes wading deeper than Least. Least Sandpiper tends to move quickly and is more likely to be seen picking at the water's edge or on drier mud. In flight, White-rumped's longer wings and white rump are apparent, while Least appears compact and lacks any white on the rump.

Both breed in northern North America, often in tundra or boggy areas near water. On migration and in winter, both use mudflats, shorelines, and wet meadows. White-rumped is a more extreme long-distance migrant, wintering farther south (Patagonia, southern South America), while Least winters from the southern US to northern South America. In North America, both can occur together on migration, but White-rumped is generally scarcer and more likely to be found in flocks of other 'peeps'.