A foot-trembling woodcock?

I had one of those encounters today where you meet a bird and then think "why aren't you running away from me?" (And luckily had a camera with me.) This nice woodcock allowed me to park my car right next to her while she went about her business. This business was actually the interesting part: instead of probing over and over to look for worms, she appeared to be feeling with her feet! In a rocking-back-and-forward motion, she would put a foot down twice before putting her weight on it and advancing one step to do the same with the other foot. 

Other shorebirds, especially Charadrius plovers, but also solitary sandpipers and others, use a "foot trembling" foraging technique that apparently helps scare inverts up to the surface. To quote one paper on foot-trembling in the semi-plover (Cestari 2009): 

• Tapping or trembling movements involve the use of one leg at time during foraging in order to expose or incite movement in cryptic invertebrates of intertidal zones and grasslands (Sparks 1961, Piersma 1996). The advantage of this technique lies in the transfer of vibrations from the foot through the substrate to the prey (Tarburton 1989). This kind of movement may also startle insects on the surface, facilitating the capture of prey by visual-foraging birds (Piersma 1996, USFWS 1996).

This sounds a lot like what I observed, and woodcock are "shorebirds" after all. (What a bad name for this group of birds.) ... But wait, after checking the Birds of North America account, I discover that alas I'm not the first one to observe this rocking, foot-feeling walk, although it does still seem to have some mystery associated with it. From the BNA account (Keppie and Whiting 1994):

• While feeding on lawns, birds seen to rock their body forward-backward without moving the head, as they slowly walk about, placing weight heavily upon lead foot. Does rocking motion and heavy foot generate vibration that causes shallow earthworms to move, a response then heard by woodcock (Marshall 1982b) or detected by its bill in contact with soil? Fascinating anecdotal reports in early literature suggest this may be so (Pettingill 1936).

Maybe so. I should have watched longer instead of getting greedy for photos!