Although you might not have looked beyond full frame yet, you might also want to consider the Olympus OM-1 and a lens like the 300mm Pro or PanaLeica 200mm (600mm and 400mm FOV respectively).
People who own both the A1 and OM-1 are finding that the OM-1 has significantly faster AF acquisition than the A1 and a higher keeper rate when AF needs to be acquired instantaneously at close range.
It also has Bird AI which will help you with finding grouse before they take flight.
Expert news, reviews and videos of the latest digital cameras, lenses, accessories, and phones. Get answers to your questions in our photography forums.
www.dpreview.com
Expert news, reviews and videos of the latest digital cameras, lenses, accessories, and phones. Get answers to your questions in our photography forums.
www.dpreview.com
Expert news, reviews and videos of the latest digital cameras, lenses, accessories, and phones. Get answers to your questions in our photography forums.
www.dpreview.com
Lastly, I'll mention that the image samples shared in these threads were shot in adverse conditions (rain, with a 2x TC) to push the AF tests to the max. Under more realistic circumstances, the IQ from this sensor and with high quality glass will be comparable to what you're getting currently (assuming you crop).
If you crop, the 300mm Pro will work great for you - otherwise you can use a brighter, faster lens like the 200mm or 40-150 Pro.
Aside from weight, weather-sealing and IBIS, there's the price advantage: $2200 vs $6500 for the A1 or $5500 for the Z9.
Edit: Oh, and an insane 50 fps with full autofocus! So even if its AF tracking is somewhat worse than that of the A1 (and it doesn't seem to be), it's mathematically impossible to get more keepers with an A1 than with an OM-1. Worth a look IMO.