I have two cels like that too from ANC, the 
sketches are nice and stuck.  But, if you dont 
mind giving up the drawings, you can dab a 
little bit of water on the backs of the sketches 
and that will weaken the paper.  Then you can 
just take the sketches off.  It'll ruin 
them..but the cel will be fine!  I personally 
dont care if I have to sacrafice the drawing of 
one or two feathers.  Just a tip tho, I love the 
feathers too!