Kiran,
Lovely image. The open beak is the icing on the cake. I would have loved a higher shutter speed. People should learn how to move close to birds and get those shots at 400mm.

Rajan,
Any lens will hunt in low light. You have to use the focus limiter switch. That reduces hunting. Also, in low light try to manually turn the focus ring. That helps in acquiring focus faster. The focus speed is also dependent on the camera body. Any lens will focus faster on the 1 series bodies than others. However, the AF system of 50D is very good. Initially I had problems in adjusting to the 40D as I was used to the superb AF of the 1DII.

Cheers,
Sabyasachi