Canon ef 70-300 IS L for bird photography?

Chesapeake Imagery wrote:

I have an RP, I had a Sigma 150-600 Contemporary and I have the Canon 100-400 Mk II. For a year or two, I shot lots of wildlife with a Canon 77D APS-C and the Sigma. It worked pretty good. Enough light was always an issue and focus speed and sharpness weren't 100% all the time, but when it hit the images were solid. I would definitely get the Sigma over the Canon for the extra 200mm reach. 300 or 400mm is not enough reach for birding 90% of the time. I tried to use the RP for wildlife, and I found it was virtually useless for birds in flight, which I don't believe it really was intended for. It also is very small and a bit difficult to handle when paired with a larger telephoto because the rig becomes so front-heavy. I would add that the Sigma is worth every penny because you get 80% of the function of the Canon 600mm f4 for 10% of the price.

I took my RP to a couple of local camera shops with the hope of trying out the Sigma 150-600mm contemporary lens. Unfortunately it's out of stock everywhere where I live but I did manage to try a Tamron 150-600mm on my RP and it was very front heavy as you mentioned. I really didn't see myself using it on a regular basis as it was just too heavy for my liking. I put the Sigma 100-400mm Contemporary on at almost half the weight and it felt very comfortable and well balanced. I have a battery pack so that also helps. The test shots I took at 400mm were very sharp and the auto focusing and Optical stabilization worked better than I expected so I decided to pick one up as it was on sale for a third of the price of the Canon version. Thanks to the feedback here in this thread I knew that the 70-300 wasn't going to do the job so I'm happy with the 100-400mm for now. I will be looking to gain more reach out of it by upgrading my RP to the R7 if and when that hit's the shelves.