I don't mind the shadow, but more the light right hand side of the photo.
It draws away a little from the subject. I like to use the flash to make the background a little less obvious. Perhaps another F stop down on the lens and slightly different lighting angle?
ALso, I am suprised at lack of deptho of field while using the flash. Hw close were you and what were your settings? I wonder if there is somethign you could have done slightly different to get better depth.
Perrhaps also a different angle?
See the photo below for a shot of a chickadee, using flash. A bit unfair because of my angle of view, directly on the side of the bird. For this, it was flash on camera. Flash off camera is a little limiting for things that move.
http://www.pentaxforums.com/forums/106132-post4.html