D3 upgrade to D4 - bird action photography

Is there anyone out there who has upgraded from a D3 to a D4 specifically for birds in flight?

I'm really interested in speed of initial focus, locking on and staying locked on?

My D3 is a bit slow to focus initially and the lock is lost very often. Not surprising for bird flight photography as the subject is so small and fast.

Can any of the sports shooters help out with their experiences, their style/requirements are probably as close to mine as I'm likely to get.

I am surprised that very little feedback exists in relation to this crucial subject for the D4, ie autofocus speed/accuracy (in normal/good light conditions)??

Many thanks,

Richard

http://www.richardbedford.co.uk