Rajan,
A couple of points to note. The quality decreases if you use converter with the 100-400 lens. At 400mm that lens is very good. The f5.6 aperture when coupled with the 2x becomes f11. You have to do manual focus. It is not easy to get manual focus on a flying bird.

When you are photographing a pelican landing, it becomes difficult focus wise due to its long beak. Unless and until the pelican is nearly full frame, it becomes difficult to distinguish its eyes.