Lens + Teleconvertor
I count myself very lucky in that I have found a reservoir, within 30 mins drive of my home, which holds a pair of breeding Ospreys. Unfortunately, although I have been watching these birds since March this year, I still haven't managed to get a crystal-clear shot.
I started out with my 40-150mm kit lens on my Olympus E420. I then splashed out on a ED 70-300mm and kept it on C-AF. I have now added a EC-20 2x Teleconvertor, which works great until you reach about 450mm and then the autofocus goes all to hell.
The annoying thing is that I'm getting the exposure settings quite good but the autofocus can't keep up with the flight of the bird. I'd love to get that 600mm close-up (1200mm in 35mm equivalent) but either myself, or the lens doesn't seem fit for purpose.
I'm using the centre (of 3) AF point & spot metering. As the distance of the birds vary AF-L is out (which is just as well because I haven't a scooby how to use it
).
I'll try the "keeping both eyes open tip" mentioned in this thread. Any other tips would be helpful. Thanks.
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-Peter 
Olympus E-420, Zuiko Digital 14-54mm (kit), Zuiko Digital 40-150mm (kit), Zuiko Digital 70-300mm, Zuiko Digital EC-20 2x Teleconvertor, Prinzflex 135mm/f2.8 (at £33, I can't grumble about the results).