[davidmanze]

f/4 is neither on the Sigma nor the Tamron available  - so that bit you have to let go, you will not get it from the zoom - and 600 to 500 mm is 0.9° difference

 

Sorry mate, this is just not worth to mention for me, especially at this FL the prime will perform great and the zoom soso, compared to it.

 

 

 I wasn't saying it was for the extra 100mm FL alone, handy that it is, it's the fact that you can also zoom out. One dreams of the bird approaching... with a fixed focal length you may get three or four frames before the bird is too close, with a zoom you quickly go to 200mm or so and continue shooting, there you get want you really want, close images of birds at shorter focal lengths where the lens is much sharper.