The sensor size on the 20D is ~1" Wide , a Robin is around 4" long so to get a frame filing shot you would need to be working at a reproduction ratio of 0.25 If we leave a bit of space around the bird we need a reproduction ratio of 0.2. This means that with a 300mm lens you would need to work at a distance of 1.8m (300mm x (1 +1/m). The same maths can be applied to any bird. Having said that, it is very easy to get this close by simply covering yourself in a standard army issue camo net (2m square) which can be bought for around £10 from an army surplus shop. After a few minutes the birds return and are completely oblivious to you, you can even move closer, they just think its a tree blowing in the wind. I've found no trouble in getting full frame shots of Wrens and Goldcrests using this technique. At this distance a wide range of reasonably priced lenses will give excellent results. I have a Sigma 28-300mm lens which I use when travelling light and it will clearly resolve the barbs and barbelles on a birds wing at around 5 feet distance.
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