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I too started the same way, with a cheaper third party lens. I then graduated to the Canon 400
F/5.6 lens. From there I moved up to a Canon 300 f/ 2.8. I now have a Canon 500 f/4 and use it mostly for BE photography. After reviewing my pictures from the last 4 years, I can honestly say there is not much difference from the 400 f/ 5.6 and the bigger Canons. Ther's no doubt that the Canon 500 is THE lens for wildlife photography. However, considering your budget, and what your using it for , the 400 5.6 Canon is a great lens. It's light and focus's super fast. There are times I still wish I had it. It,s great for stills and perfect for BIF. Remember most of Us turn off our is for moving birds, for faster focus, so I don't think you'll miss it that much.
In summary, get the Canon 400 f/ 5.6 . There are a lot of great deals on used ones right here on this forum.