FYI all, what you need to look for in distinguishing a Gyrfalcon from a
big female Peregrine is the two-toned under-wing of the Gyr.  It's visible
in pretty much all color morphs, even -- so I'm told -- a white Gyrfalcon. 
In the darker birds we generally get here, it will generally be pretty
obvious if you can get a good look, and it nails the ID.  In the excitement
of seeing a honking big falcon, one can forget to look for this until it's
too late.  My ancient computer can't play the video, unfortunately, to see
if this characteristic is visible in it.

Jane Stein
(Shoreham)