"Tiny active birds of tree branches,... with a characteristic habit of nervously flicking the wings."
-- The Birds of Canada, 1979 by Earl Godfrey.
In our previous identification post about the separation of Ruby-crowned Kinglet and Hutton's Vireo we concentrated on the color and shape of feathers, bill, legs, and feet.
Birds also have many interesting behaviors that help separate them. These are rarely mentioned in the field guides--unless conspicuously obvious. Thus field guides typically mention the bobbing and teetering of Spotted Sandpipers, the hovering of Rough-legged Hawks, and the plunge-diving of Belted Kingfishers.
The nervous wing-flicking of Ruby-crowned Kinglet also is mentioned in most field guides.
This innate (instinctive) behavior is like a constant tic. Every couple of seconds the kinglet flicks its wings, as in the photo above. When agitated, it may flick its wings twice in a second. Combined with its constant movement--rarely does a kinglet sit on a branch more then 4-5 seconds--this tiny bird gives the impression of being in a constant state of nervous hyperactivity.
This isn't to say that the look-a-like Hutton's Vireos never flicks their wings, because they do--many small birds do--just not anywhere near the same degree as the kinglets.