Sorry Widowmaker I don't know how your query got past me and everyone else for that matter. Bob is definitely right in that the closer you can get to subjects, the better the quality of images. most of mine seem to get taken at any where from 30 yards up as much as 90 yards, but it really does depend on having the right conditions. A cool bright sunny day with no wind or heat haze seems to be the best - with the sun behind you. I have had reasonable shots at a lot greater distances too, but they are the exception.
Realistically for consistent good results I'd agree that 20-60 yards should be about right, perhaps a bit more with your scope being a bit 90mm.
* Oops I think we posted at the same moment.
I agree that afternoon sun gives the right angle for decent lighting conditions. I can't answer your query about the manual focus as I only use the autofocus which I lock by half depressing the shutter button and then re-focus using the scope adjuster whilst looking through a 2x magnifier. I find it is far less fiddly doing it that way.
I'd say your Chickadee shot came out just fine. The only additional thing I'd have done would be to maybe use the five focus brackets and on this shot have selected the left hand one which would have given the sharpest focus on the head and eye. I always try to get the eye as the sharpest part of the bird. By locking the autofocus in the fashion I describe it's pretty easy to use the scope adjustment to focus on the bit you want.