Pete, the kenko certainly helps. But the closer we get, the closer we want to get. 
There IS a way actually and I am sure lots of birders do that though they never mention it. Notice how a ton of shots are taken in backyards? Obviously the birds are being lured there with the feeders. I am pretty sure they even "tailor" the backyards to get the best views. That is the lazy but will get you "the" shots method!
Anyway, one guy did mention that he scouts around for a good location, lighting etc and looks for isolated branches - any place he can get nice clear shots. He then sprinkles some seed on the ground to bring the birds in and then hangs a feeder on some isolated branch. He probably brings along coffee, folding chair and other comforts. Easy! The slightly less lazy and get the shots method!
I took up outdoor photography for the exercise and might as well be productive at the same time. Much better in my opinion then being stuck in a gym. Multi-hour walks/hikes are normal. In my situations, reach is important but so is weight. I could have gone with any of the zoom options but settled on our lens + the TC as this gives us a reasonable reach and not the bulk of longer and heavier equipment.
Here in Hong Kong small birds predominate so rather then lusting for reach, will just concentrate on "things and their environment" type shots. Back home the lens should handle the bigger birds well.