Pralay,
Is this a crop or full frame image? I think it would be been good if you had used f8/f9 aperture here. That Sigma lens works well when stopped down by one stop. Also in this case, the depth of field is not sufficient to cover both the vultures. The vulture on the left seems to be closer to you, whereas the focus is on the other vulture. So the overall image looks soft. Thanks for sharing.

Cheers,
Sabyasachi