I've seen basic-plumaged shorebirds often in Spring, and had always assumed
that they had started migration before molting. This happens in some other
birds (mostly passerine examples). But increasingly, basic plumaged birds
are being found here late into Spring, leading many to wonder if they migrate
at all.
As to your second question,it depends on the organization of the flock. I
suspect a fairly organized flock would be called a 'braid'.
Rob Thorn, Tacoma (rthorn at ups.edu)