For bird photography with the Canon RF mount, I highly recommend the Canon RF 100-500mm F4.5-7.1L IS USM lens. Some key benefits:

• Excellent focal range from 100-500mm, giving you flexibility for both close-up shots and birds in flight
• Relatively compact and lightweight for a super-telephoto zoom
• Fast and accurate autofocus for capturing birds in motion
• 5-stop image stabilization for sharper handheld shots
• High image quality throughout the zoom range

It's a versatile, high-performance lens ideal for bird photography. Pair it with a body like the R5 or R7 and you'll have a winning combo.

I've been shooting for years and honestly I'm pretty satisfied with how things turn out these days. No complaints really about the sharpness or the tech on these new bodies. But honestly... the distance thing just kills me every time. You think you've got enough reach and then a raptor sits just far enough away to look like a tiny speck in the frame. I think I read somewhere that even the guys with the massive prime setups deal with this, so maybe there's no winning. IIRC someone told me it's more about the bird's mood than the gear anyway. It's just so frustrating when you finally find a rare songbird and it stays in the thickest brush possible or just out of range. Just venting because the struggle to get a clean shot is real no matter what you're carrying.

tl;dr: The Canon RF 100-500mm f/4.5-7.1 L IS USM is the most reliable pick for an R5, but you gotta be careful with how the extenders work. Just saw this thread and thought id chime in as a long-term user. If you go the 100-500 route, you might want to consider the Canon Extender RF 1.4x for those tiny songbirds, but please be careful. The lens actually wont retract below 300mm when the extender is attached. I was pretty nervous about that at first because it makes the whole kit a bit more vulnerable in the bag, so just make sure to handle it gently and dont force the zoom ring. Also, for raptors in low light, that f/7.1 or f/10 aperture with the extender can be tricky. I would suggest keeping your ISO a bit higher than you think you need. I also highly recommend grabbing a B+W 77mm Master Clear MRC Nano Filter just to safeguard that expensive front element from dust or accidental bumps. Its better to be safe than sorry with gear this pricey. Let me know if you need more help with the setup!

The new released RF 200-800mm lens is another great wildlife lens, but it is hard to find, out of stock everywhere.

I also recommend the RF 100-500mm L lens.

RF 100-400mm lens is another great choice for shooting birds. https://www.bhphotovideo.com/c/search?Ntt=Canon+RF100+400mm+F5+6+8+is+USM&BI=8941&KBID=10361&SID=12345&DFF=d50

When comparing the 100-500mm L lens and the 600mm fixed focal length lens, budget considerations play a significant role. The 100-500mm, being an L-series lens, comes with a higher price tag but offers superior optical quality and versatility. Its zoom capability provides flexibility in framing, while the 600mm offers a fixed focal length.

The 100-500mm boasts exceptional optics, superior stabilization, and faster performance, allowing for greater control over depth of field. Although heavier, its build quality is superior, featuring weather resistance that the 600mm lacks.

These lenses are fundamentally different in their design and intended use. The main reason for comparison often stems from the price advantage of the 600mm. However, the 100-500mm's superior features and versatility justify its higher cost for those who can afford it. Ultimately, the choice depends on individual needs, budget constraints, and the specific shooting scenarios one encounters.