When I got home from work today I came inside and noticed a Hermit Thrush on my lawn. The bird appeared to be shivering, which I thought was perhaps the cold weather (it was about 5 degrees Celsius). However, I quickly realized the whole bird wasn't shaking but it was just very quickly tapping the ground with one foot. I watched the bird (and noticed another Hermit Thrush doing the same thing) for about 45 minutes off and on. I remember seeing the same behaviour once before at Bird Studies Canada Headquarters by a Semipalmated Plover and Ron Ridout at the time told me it was a foraging behaviour used to draw prey up to the surface. I've heard second hand that Wood Turtles actually employ a similar technique!
Anyways, I did some searching and found this paper from the Wilson Bulletin that documents the behaviour in some of the other Catharus thrushes and also suggests that the "foot quivering" is a foraging behaviour - something that seems well supported by the video I took: