Well, give me an optical viewfinder any day for BIF! My BIF kit, Canon 60D plus 400mm f/5.6, weighs 2.0 kg. or less. It is long, though. BIF shooting is exceptionally demanding of a camera/lens' smooth and fast operation.
Funny you use a 60D. I just sold mine because I had trouble focussing on a stationary bird, let alone one flying past. Those dirty great focus points just wanted to grab focus on the nearest branch, the birds rump ... anything but where I wanted focus. Even with my lovely 70-300L, it would frequently rack focus up and down while the bird stifled a yawn and flew off! And that was the best of several long lenses I tried on that body. After coming from an Olympus E3 with equally lovely 50-200 lens, which just instantly focussed exactly where I wanted to without a glitch, it was very disappointing. That is actually why I moved to an EM1 and less lovely 75-300 lens. It has a tiny and deadly accurate focus point which never misses and the combination weighs nothing by comparison, which means it is with me far more often. Turns out I'd rather take a slight hit on image quality than not get the photo at all (though the Canon combination was quite wonderful when it all came together).