Ultimately, what you need to increase DOF at a given f-stop is a reduced image format (size of the bird's image on the sensor), either with cropping or by using a smaller format sensor. You have effectively noticed that with your comment about detail loss at 2 meters.
This might require increased lens and sensor resolution, in the sense of "lines per millimeter".
Moving back and increasing focal length to get the same framing does not help; you get about the same DOF from that same f/16.
As you noticed, you do gain DOF at teh cost of resolution if you use the same focal length from further back and then crop : doubling distance and then cropping to the same framing, which involves enlarging twice as much, roughly doubles DOF.
Almost equivalent DOF gain and detail loss would be achieved by staying at your current distance, reducing focal length and again cropping.
So if the 20D does not allow enough cropping and enlarging, higher sensor (and lens) resolution is needed, requiring smaller pixels. The three current choices for distinctly smaller pixels are
a) any compact digicam (their famous abundant DOF can pay off here!),
an Olympus E-300, or
c) a Nikon D2X.
The first two effectively build-in the cropping, as does the D2X if you use its 2x crop mode.
My bet is that for the sake of having the lens resolution keep up with the increased resolution needs it is better to use lenses designed for smaller formats rather than cropping from larger format lenses, and it is far less expensive too. On the other Nikon has some fine lenses if you can afford that option.
One trade-off is that the usable ISO is lower with the smaller pixels. Hummingbird photography probably requires flash in most situations!