Some birds follow moving swarms of army ants in the tropics. As the ants march along the forest ...

Some birds follow moving swarms of army ants in the tropics. As the ants march along the forest floor hunting insects and small vertebrates, birds follow and pick off any insects or small vertebrates that fly or jump out of the way of the ants. What is the specific benefit relationship?
A) Birds benefit from the association, but have no impact on the ants.
B) Birds benefit from the association and harm the ants.
C) Birds and ants benefit from the association.
D) Birds do not benefit from the association, but the ants do.
E) Neither birds nor ants benefit from the association.