Unfortunately, if you stick with those standard SanDisk Extreme Pro V30 UHS-I cards, you are gonna hit a wall pretty fast. I tried that with my R6 II when I first got it and it was honestly such a letdown... the buffer takes forever to clear after a long burst of bird action. When you are shooting at 40fps, those V30 cards just cant keep up and you end up standing there watching a red light blink while the bird flies away. Its happened to me more than once and it feels terrible to lose those frames. I generally lean towards being more conservative with my storage choices because I really hate the idea of a card failing or corrupting. I have had some disappointing experiences with cheaper high speed cards that overheat or just stop working mid-shoot. If you want reliability without spending quite as much as the top-tier ProGrade stuff, I have been using the Kingston Canvas React Plus V90 128GB SDXC lately. It is surprisingly solid for the price and usually comes with a reader, though I still worry about long-term durability compared to the rugged options. For absolute safety and speed, I usually suggest the Sony SF-G Tough Series V90 UHS-II 128GB. They are pricey, but they are built like a single piece of plastic so there are no ribs or switches to break off. It is a bummer the price jump is so steep, but for birding, V90 really is a different world. Just make sure you have a fast reader too. Let me know if you need help finding where they are on sale right now.

Saw this earlier but finally replying. You might want to consider that the interface matters just as much as the speed rating. The real bottleneck for an R6 II is usually using UHS-I cards instead of UHS-II. I learned this the hard way during a shoot years ago when my camera locked up for a full minute while the buffer cleared. Its beyond frustrating watching the action happen while your camera is basically a paperweight. I would suggest ProGrade Digital SDXC UHS-II V60 Gold as a solid middle ground. It has the extra row of pins to actually move data fast enough. Make sure to avoid the standard V30 cards for birding or youll hit a wall after two seconds of shooting. The V60 is usually the sweet spot for cost versus performance. Be careful tho, even with fast cards, you still gotta manage your bursts properly or you will fill any buffer eventually.

I totally get that stress. I was in the exact same boat when I first got my mirrorless and started shooting wildlife. I was terrified of missing a shot during a local eagle migration, so I almost dropped a fortune on those ultra-fast cards. Honestly tho, Ive been using the Lexar Professional 1667x V60 SDXC for over a year now and I am super satisfied with how it handles. Sure, a V90 clears the buffer a bit faster, but for most birding, I rarely hit that wall unless I am holding the shutter for way too long. You can save a ton of cash sticking with a solid V60. I also have the ProGrade Digital SDXC UHS-II V60 Gold and it works well without any complaints. For the price of one V90, you can basically get two of these and have a backup, which is way safer for your data anyway.