Wildlife & Birding Upgrade what be better? Nikon z 600 f/6.3 or Nikon z 400 f/4.5

I currently use 180-600mm for wildlife and birding... I feel like its time to upgrade to a prime lens... I find myself mostly birding (like owls and eagles etc)... I do like doing mammals but not to many by me... but I just moved and yellowstone or grand tetons not to far from me so hopefully I can get more mammals... I will also be in africa over the summer and be doing a safari (Namibia)... I own a 1.4x teleconverter also (which in theory could pair with 400 nicely or the 600)... what would all recommend? Thank You!

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Some call it GAS, I say I'm a collector.
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D750 with Nikon 200mm F4 macro, this is my favorite macro lens, so easy to use, no flash light needed!

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Sigma 500mm with birdies

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Guess Who
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Okay, I hate to do this, but I have to ask which Nikon this is.
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D750 with Nikon 50mm F1.4, my favorite lens to shoot in the dark!
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Mushroom hunting with my Zf + z24-70mm f4 S!

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Some shots with D700 and 50 1.4d

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nyc thanksgiving scenes on the Nikon zf + fe2

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A trip to Granada [Nikon d500+nikkor 10-20 4.5-5.6]

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