英文文法一問

Who chased the bird? is better. It is because "chase" is a verb...
it can be a v.t. or v.i. .and it means 追逐;追捕;追蹤,趕出,驅逐...
e.g. The cat chased the mouse.
e.g. She chased the children from her yard.


however, What chased the bird?
is also OK. Since, we don't know what is the subject in this sentence,
the subject can be an animal or a person.
If it is an animal, we shold use WHAT,,
If it is a person, we should use WHO..