Flocking behavior is the behavior exhibited when a group of birds, called a flock, are foraging or in flight. There are parallels with the shoaling behavior of fish, the swarming behavior of insects, and herd behavior of land animals.
birds also flock together because there is safety in numbers and smaller birds working together can fight off larger birds, as well as enjoy the economic and efficient aerodynamic advantage. Of course very strong birds of prey do not need to flock together because they have the strength and agility on their own to protect themselves. Likewise they are generally adapted hunters and can easily get a meal to give them more energy anytime they are hungry.
Some of the reasons birds flock together and other birds do not flock has to do with Evolution and Breeding. The stronger birds will survive and be able to feed their offspring better because they are better hunters and their offspring will grow up stronger and pass on those genes more often.
