Originally Posted by Joel Eade
Are you hand-holding the camera? That would seem to me to be the main reason to want a higher shutter speed. Have you tried setting up some sort of support, like a tripod? The reason I ask is that your lens will have a VERY shallow depth of field, especially fairly close. 420mm, f/4 (the lens apparently wide open with a 1.4x extender), at a distance of, what, 10 ft? (You'd have to be close for the bird to be that large on the image.) DOF would be something like 0.02 ft, or 0.24". At 20 ft, it would be 0.1 ft or 1.2", but only 0.6" in front and behind the focus distance. That seems to agree with the last photo. While the focus is perfect on the eye and beak, the bird's body fades away to a blur about half-way down. F/8 would double the DOF. A suggestion: try raising the ISO to 800 & using aperture priority, rather than shutter priority, then different apertures (and speeds, of course) to see if it makes a difference. Most of the articles I've read on photographing birds have not suggested shutter priority, probably because, unless the bird is moving, the shutter speed doesn't matter to them as much as controlling DOF.
Another way to improve the light might be to use fill flash. Your lens is a perfect match for a decent E-TTL II flash (e.g., 430EX, 580EX) and a Better Beamer flash extender, depending upon the distance. Lots of bird photographers use fill flash (Nate does, as I recall). Putting the flash on the hot shoe will work, but raising it a bit higher on a bracket can be better. You can find good info about the Better Beamer by Luminous Landscape, the Strobist crew, Moose Peterson, and Ralph Paonessa's site. (Ralph sells the Better Beamer and has the best information I've found.) It is recommended (and sold) Art Morris, probably one of the best bird photographers around. The Better Beamer is sold by B&H, Adorama, Naturescapes, and other sources.