Dug up from this past March...
Driving along Lake Musconetcong I happened upon a mated pair of downy woodpeckers. They were very close outside the car window - close enough that my Tamron 400/4 65B + Pentax A 1.4x-L combo gave me too much magnification, to the point were sometimes I couldn't fit one of their little bodies in the frame. But they were moving around so quickly I daren't stop and remove the TC. And they were too quick for me to really focus accurately - it was "spray and pray" for about 60 seconds until they were gone.
I never really nailed focus exactly where I wanted it, I was most often half an inch or so off. But I may never get so close to the little birds again, so I have to be happy with what I got.
(cross-posted in the Adaptall thread)