Whilst spending a day with Feathers and Fur in Berkshire I managed to catch Acer, a Peregrine Falcon whilst he raised his feathers, and shook. His head remained perfectly still whilst his body rotated and shook as if an engine or rocket was getting ready to take off.
This is called a rouse, and is done to shake the feathers into place, as well as to remove dust and debris. A captive bird will only rouse on a glove if they are comfortable with the situation. Unfortunately this photo was only a runner up however if you would like to see the other photos taken that day please see the link below.