there is a sigma in the adorama overstock section 100 ish mm to 400mm. It is a jump in price from the tamron you are looking at but the most reasonably priced that I found when I was looking a couple months ago. If you really are going to be shooting birds then you may want more than 300 mm.
727109 Sigma 135-400mm f/4.5-5.6 DG APO Aspherical AutoFocus Telephoto Zoom Lens with Hood & Case for Pentax AF Cameras
this is no DA* lens but then again the tamron you are looking at is not either. I think you will be pleased with the extra reach of a lens like this and you really can get the subject to fill the frame. I have never used this one so I can not speak of its quality but I think it should meet your needs.