Who did the hippo see first?
1. He first saw the bird.
2. First he saw the bird.
3. He saw the bird first.
4. He saw first the bird.
(Which ones are the suitable answers? Which is the right position of 'first'?)
Since birds are not humans, you should use "What" instead of "Who" (which is incorrect anyway! -- it should be "Whom").
What did the hippo see first?
The best answer is #3 ... the others are OK, but awkward sounding.