I like that the subject is sharp, the exposure is correct and the background is blurred nicely. Really, these are the most important elements. That said, there is always room for improvement, right?
I do not like that the tail was cut out of the image. It may seem trivial, but it is a generally a good idea to either take a photo of the entire bird, or just the head. When little things (toes, tail feathers, wing tips) get cut out, it disrupts the framing of the photo, as the subject goes all the way to the edge, but only at one small part of the image.
Also, I am not a fan of plastic feeders in my shots. The tree idea is a good one.
Finally, the back lighting dulled the color in the feathers. You could throw in some fill light from your flash. If you are shooting through a closed office window, this will obviously not work. Make sure to take the flash off camera, as you want to maintain detail (i.e. tiny shadows) in the feathers and this is best achieved with your light hitting your subject at an angle to the lens.
Just my thoughts.