[Rover]

The 300-600mm F4 sounds about the pinnacle of where birders want to be in focal length
all for a price of around $/€ 6,000 .... for most F4 lenses you are looking at double that ....

  next question is will it be available for the Z mount?

It already is (inasmuch the lens itself is available)... with the tiny sliver of an E-Z adapter.

Anyway, I think most birders would probably find 600mm too short. Therein lies the rub, as the issue of TCs in E mount is notorious.