I tried adapting an older EF lens on my R7 to save cash, but it felt back-heavy and the autofocus hunted. You might want to consider the Canon RF 100-400mm f/5.6-8 IS USM instead. Why its safer:

• Native RF mount means zero AF lag
• Super light for hikes
• Often on sale refurbished Just make sure to get it with a warranty.

Re: "I tried adapting an older EF lens on..."

• I actually ran the Canon EF 70-300mm f/4-5.6 IS II USM on my R7 for a few months. That specific EF lens has Nano USM, so autofocus is actually blazing fast. But once you add the Canon Mount Adapter EF-EOS R, it gets kinda front-heavy. If youre hiking, the native Canon RF 100-400mm f/5.6-8 IS USM is just way easier on the arms.

Like someone mentioned, adapting lenses can sometimes be a headache, but after years of shooting wildlife, reliability is my main focus. If you want rock-solid reliability, look at a used Canon EF 400mm f/5.6L USM. Its an older lens but built like a tank. The AF is incredibly fast and accurate, even on the R7. The downside is no image stabilization, so you need fast shutter speeds, but the optical quality is legendary for the price. If you want native RF, check out the Canon RF 600mm f/11 IS STM. Its insanely light for hiking and gets you much closer to tiny backyard birds. Pros are the weight and native AF tracking reliability. The major con is the f/11 aperture, meaning you need decent sunlight. Tbh, both are solid, but for pure reliability and ease of use on hikes, the RF 600mm is hard to beat. Drop a reply if you want some tips on shooting at f/11, it is easier than it sounds.

Building on the earlier suggestion, I'd grab the Canon RF 100-400mm f/5.6-8 IS USM refurbished. In my experience, skipping the adapter saves money and weight, which is crucial for long hikes.