You haven't given us any idea of how advanced you are as a photographer, and how and what you shoot besides telling us that you shoot birds and animals. So we have to guess as to what you shoot, and how.
- If you're coming from a 20 MP D7500, then the 46 MP files generated by the Z8 are 60% larger in terms of how much storage space they take up on a memory card or disk drive.
- Birds and animals often move, and sometimes they move fast. Think, for instance, shooting a bird while it's in flight, or shooting a sequence of an osprey diving into the water for a fish. The best way to get at least a few decent images under such conditions is to shoot in burst mode. Steve, in the guidance he posts on Backcountry Gallery (BCG), advises to shoot the maximum frame rate when doing bursts. On the Z8 that is 20 FPS (20 frames per second) in RAW. If you do JPEG, the frame rate can be even higher. Depending on the scenario you may not get all the images in a burst perfectly sharp - the camera may miss focus on a few frames, and that is why you want to shoot a burst to improve your chances of getting that real keeper.
- The write speed to SD cards is slower than the write speed to a fast Cf Express card. This means the bottleneck caused by a slower SD card will fill the camera buffer faster, and that then slows down the camera's frame rate. You can shoot bursts for longer on a fast CF Express card than on an SD card before the buffer fills and slows down your burst rate.
- A day of hectic shooting in burst mode with a Z8 will at some stage fill a 64GB memory card and force you to swap out the card in camera with a fresh one. And it's during the swap that you may miss the shot of a lifetime.
It's for the above reasons that it just makes sense to use fast CF Express cards with more storage capacity as your primary storage on the Z8. Also, being built more robust and in stronger housings, the CF Express cards will be able to handle more abuse without failing. On the Z8 I wouldn't consider anything smaller than 128 GB. My smallest CF Express is a 150 GB card, and I have another 325GB and one 512GB card.