Spotted Sandpiper

The Spotted is the most common inland Sandpiper although their behavior doesn't otherwise set them apart from other Sandpipers. Aside from laying more eggs, I wonder what gives a species like this such an advantage over other similar species. There doesn't seem to be any clear superiority to the Spotted Sandpiper over the Least Sandpiper, but the Spotted has managed to spread its range over the entire North American continent nonetheless.
This lone Spotted Sandpiper (does that make him a Solitary Sandpiper? >haha birding joke<) is not in its breeding plumage and therefore is without the heavy spotting on its flank, but the yellow legs, size, white shoulder patch, white eye ring and slight yellow on the bill, as well as its presence in Phoenix, all still give it away.