Semipalmated Sandpiper

This is particular sandpiper and Sanderlings are really hard for me to tell apart. Even now, I’m mostly trusting my father’s (and Merlin’s) IDs for which photos are of which! Doesn’t help that they can both vary from relatively monochromatic to somewhat ruddy. Both run around like little guys on the shore, both have similar patterns on their back. I think their size is slightly different, but in photos with only a couple birds it’s hard to use it! Regardless, I like these guys and the fact that there’s a Samipalmated Sandpiper and a Semipalmated Plover, but the Semipalmated Sandpiper looks like Sanderlings is extremely silly to me.
Wikipedia
Audubon