samchang

Po Toi, 4May. Is it just a Common Sandpiper? The color and pattern on the back made me feel it may not be, also the black patch under the wing seems strange.

HFCheung

Yes, it is.  Very few waders have barred pattern on the back.

ajohn

The upperpart pattern may look different because it is getting breeding plumage, so is more barred on the back than they would be in winter. The dark patch under the wing is shadow on the rump visible because the wing is slightly raised.

samchang

Thanks all for the explanation!