Why do birds fly in groups?

Birds travel in flocks for protection. They flock to avoid predators. More eyes watching means it's harder for a cat or a hawk to catch the birds off guard. And they flock to find food. Again, more eyes makes it easier to search. Also, the flock can form around the food. One bird sees the food, a second bird sees the first one eating, and joins in, then a third bird sees the first two, etc.