What lens to shoot birds/wildlife with my Canon EOS 50D?

[Saluki]

I have a Canon 50D and 2 lenses - 18-135 and a 70-300. I'm thinking of adding a telephoto prime lens in the  500-600mm range. Is there anything out there that I might find on the used market in the $500 - $750 range that would be a significant increase in photo quality?  Would the Canon RF 600mm f/11 IS STM work with my EOS 50D?

[ebiggs1]

"The 300mm is great too and has image stabilization, not as long a reach but..."

Oh, man, you are so right. I love the ef 300 mmf4L. Here's the kicker, buy the ef 300mm f4l and the 1.4x tel-con. You end up with a 420mm f5.6 that has IS. The ef 300mm has even better IQ than the ef 400mm so it has room to give the tel-con and still be great.