This one's on a feeder - not nearly as good as a tree, for a wild bird. But the feather detail (in spite of noise) tells me what I need to do is get closer with the long zoom for the tree shots, and try to get away from direct sun on the bird. Those two things in combination aren't easy, since there's a much higher incidence of blur with bird movement, low light, and long zoom. So Geoff I can really appreciate that you managed to get any good ones at all, since I suppose most of those National Geographic type shots are from tripods, and the wait times are pretty long.
Definitely on a tripod; I've tried shooting the occasional bird hand-held with a long lens, and results just aren't worth the trouble.
Here are two of my woodpecker shots; these were taken with a 500 f:4 and 1.4x teleconverter on an Eos-50D, for an effective focal length of 1120 mm. That may seem a bit over the top, but it permitted me to get enough magnifcation for a worthwhile image, while staying outside the bird's 'circle of fear'. It was about 40° F and absurdly windy, something like 40 mph gusts, so many of the shots were less than sharp simply due to wind vibration. I lucked out on the light; the sun was just breaking through some haze behind me, so shutter speeds were acceptable at ISO 400 and contrast was reasonable.
We live in ideal bird habitat, with about 15 acres of abandoned apple/cherry orchards and several pastures. The only problem is that my real job is about 90 hours a week, so sometimes I lack the energy to get out there. Kudos to my wife for pitchforking me out of bed to see this bird.