Which lens? (1 Viewer)

Hello,
I choose lens for photographing birds. I have D90 + Tokina 400mm AT-X and
now the resulting pics are not too good...
I decide between Sigma 100-300 f4 + TC and other zoom Sigma (50-500, 120-400, 150-500). Importatnt is image quality on the end of focal length.
Same reviews of 100-300 are not too good (http://www.juzaphoto.com/article.php?l=it&article=44)... Can be it worse with TC than other zoom Sigma (50-500, 120-400, 150-500)? Nikon 300 f4 AF-S is too expensive for me...
I am concerned that this lenses will not be much better than my Tokina (http://www.flickr.com/photos/michalhykel)

Thanks for help

you would be better off with a nikon 300mm f4 prime ,much better quality and you can use it with a 1.4 tc or a 1.7 tc and still get extremely good photos ,look at my flickr stream for proof

You shot all of these with the Tokina 400 ? They are excellent

What exactly is the problem with these photos ? You wont get better with any of the lenses you mention, and the 3 sigmas do not really do well with TC's


http://www.flickr.com/photos/michalhykel/8189902289/in/photostream/lightbox/

http://www.flickr.com/photos/michalhykel/8190984880/in/photostream/lightbox/

http://www.flickr.com/photos/michalhykel/8189898939/in/photostream/lightbox/