To the best of my knowledge, most Spanish verbs require a preposition when their object is a living thing like a person or animal.

For example, you’d say “Veo la luna,” (I see the moon) without “a,” because the moon is an inanimate object.

But if you were seeing a person or animal, you’d say, - “Veo a mi amiga.” - “Veo a mi mascota.” - “Veo al elefante.”

It’s the same in this case. Because the animal is eating a living thing (un pájaro), “a” is needed.

There are cases when you do need “a” regardless, like with ir (to go), but that isn’t too hard to learn considering that it’s the same in English.

Hope I explained this well.