Tripods, birds, and shutter speed
The rule of thumb when shooting handheld is to use a shutter speed of at least 1/focal length (assuming no image stabilization.)
First question: on a crop body such as the Canon 50D is this before or after accounting for the magnification factor? e.g. with a 300mm lens should the shutter speed be at least 1/300s or at least 1/480s?
Second question: how slow can you go with a long lens on a non-locked down tripod? I.e. the birds are not stationary and you can't take as much time as you need to frame the bird perfectly and then shoot a few seconds later with a cable release. (Ditto assume the camera is not prefocused on a feeder or obvious perch.) Assume you need to be tracking birds in the wild through the viewfinder and shooting as they move around to some extent. Have we reached the point where subject motion is going to limit the shutter speed more than camera shake? And if so, what's a typical minimum shutter speed for songbirds?