Birds seldom stand still. They move all the time. Even if they only move their head can ruin your photos. I suggest you set your aperture at the widest opening so you will have highest speed. I consider speed slower than 1/400sec not suitable for wild birds.
See my birds pictures here:
http://www.birdforum.net/pp_gallery/....com&password=
Most of the bird pictures here were taken without tripod with speed faster than 1/400sec.