Hi All: There was one solitary sandpiper at Fisher Pond on Vashon Island Sunday (25 April). It was within 50 ft and through the scope I could see it quiver or vibrate its feet as it walked along floating mats of vegetation. This is different from the paddling or puddling done by gulls and killdeer that looks like the bird is running in place to stir up critters. With each step the solitary's foot and leg vibrated like a Sonicare toothbrush as it put the foot into the water. I imagined I could hear it the hmmmm.

Looking on google there is a is a paper on semipalmated plovers with great picture of a bird in a foot vibrating posture http://www.scielo.br/scielo.php?pid=S1676-06032009000400036&script=sci_arttext. The behavior appears to be fairly well known as foot trembling, especially in plovers. Birds of North America for the solitary sandpiper cites Forbush (1916) "...wades in stagnant ponds or ditches where it stirs up vegetation by rapidly yet delicately moving foot to rouse insects, catching them as they attempt to flee ."
Gary Shugart, Slater Museum of Natural History, Tacoma and Vashon Island