The 100-400L can sometimes produce donut oof highlights, which are typical of a mirror (cat) lens, and some of those oof highlights in Dod's robin shot seem to be just verging on becoming donuts.The effect seems to be a random, occasional thing, but is probably caused by a combination of chosen focal length, the nature of the background, and even the aperture. It's an effect not common to the 100-400L either, other zooms can produce the effect. It's certainly not camera shake as the bird and the twigs in the foreground are sharp.
You ideally shouldn't be touching the camera at all when you take shots on a tripod - that's the whole purpose of using a tripod, so release the shutter with a remote release, rather than touch the camera and induce camera shake.
The technique Matt describes for pressing down on the lens etc. are all accepted techniques when tracking birds fliitting about and not keeping particularly still, when you really need the 'hands on' approach to have any chance of being able to manouevre the camera and get a sharp shot when the opportunity presents itself, but wherever possible use a cable release.
