Head shots of birds in the wild? Drop $7K on a DA 560 and learn how to get within a few meters. (Minimum focusing distance 18.37' (5.6m))
The Sigma 150-500 like I have, $800 if you can find one (discontinued), has a minimum focus distance of 7.22' (2.20m).
At that distance it isn't macro...
And it's not a lens you are going to use a Raynox on...
Cropping works well if you have enough MP.
This is cropped to 4.6 MP with the K-5. Distance about 20'. (Maybe less)
This is cropped to 9.6 MP on a K-3. Taken through a window. Distance about 10'.