Male American Redstart (Setophaga ruticilla) displaying at the Marine Nature Study Area in Oceanside, NY. The bird seemed to be foraging, but combined this with a zig-zag stroll, fanning out its tail and prancing some 20 meters up a woodland trail all the way to my feet!
Update: I was informed by an ornithologist colleague that they do this to flush their insect prey.
In any case, he approached me with little concern, allowing me to take dozens of pictures. Unfortunately, few were as sharp as one would like, as my autofocus insisted on zeroing in on the warbler's tail, combined with a dark environment.