There are some alternatives, which are smaller, lighter and a great deal cheaper but they will challenge your skills more.
The Tamron SP 500mm lens, on a M43 body, is equivalent to a 1000mm lens on full frame. This is capable of good results
if you can hold it steady and focus it accurately. Whether it will work for a bird in flight, I couldn't say, but at a price of between £60 and £150 on the open market it's a good starting point.
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It's fine for birds on a branch, I don't know about birds in flight though...
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