A minimum 300mm focal length really.
What system are you using?
I have a canon 7d and a 70-200 f2.8 with an extender. I hired a canon 28-300 from Lenslocker and found it quite heavy over a period of time. I'm not sure if there's a lighter option.
How about you? What are you using?
Hi the 70-200 f2.8 is a lovely lens and performs well with a 1.4TC, I havent tried it with a 2x TC however.
Have you thought of the following
Canon 400mm prime f5.6 or the 100-400 zoom
The Sigma 150-500 is also very good value for money and only marginally les sharp than the canons above if you get a good version - QA/QC issues can be a problem with this lens.
I mainly use a Canon 100-400 or a Sigma 150-500. I have other other lenses but these are the two I use most often I guess. I also use the Canon 7D.
I shoot Nikon so my comments may not interest toasty but other Nikon users might find them helpful.
I have two long lenses and I like them both. The first long lens that I bought was the Tamron 200-500mm , it is quite light for lens of that length but lacks a focus limiter and stabilisation. I have taken good shots handheld but in poor light it really needs a tripod.
I recently bought a Nikon 300mm and focusing is much quicker than the Tamron and it has focus manual over-ride which is excellent when birds are not out in the open.
I have a Nikon d3100 and have the nikkor 55-300 lens as my walkabout. I do hope to acquire the sigma 150-500 in the near future though.....
If you want close-up shots of birds, particularly small birds, then it is difficult to get lightweight kit that will do the job. I use a Nikon D5100 and the Sigma 150-500 lens but together, it is relatively heavy - you can see examples of my images in the Flickr link below.
Having said that, I meet people with very expensive Nikon and Canon lenses that are capable of fantastic photography but many are much heavier still. I watched one guy pack up his kit into a large four-wheel trolley in order to get it back to his car! It is certainly worth trying out various lenses but the real test is walking around with it in your hands for 2-3 hours.
The final answer depends on what you can afford and how you are going to use it. If you want to set up in a hide on a regular basis and can afford it, you can go for an expensive branded lens that will get up really close to the birds (but is quite heavy). If like me, you can't afford such lenses and you want to walk around looking for birds, then you need something lighter (and cheaper - thus the Sigma 150-500 or similar). Andrew is a good model to base your decision on because he uses a variety of lenses for different purposes in different situations.
Good luck with your search.
I recently bought my first DSLR from jessops, a Canon 1100D. Entry level body and a Tamron 70-300mm lens. Then bought the 2x convertor but was still not happy with the results, just bought a Sigma 150-500mm lens and it is superb by comparison. It is a lot of lens to carry about, we did 4 hours of walking at Rainham Marshes last weekend but the pics were well above anything I have shot before so the mild neckache was well worth it for the results.