Why do birds flock?

All different kinds of birds from sparrows to hawks form flocks. They can number in the single digits or be composed of thousands of individual birds seemingly acting like one cohesive organism. The main benefit gained from flocking, especially for small birds, is safety in numbers. Although overall a flock is much more conspicuous and easily targeted by predators than a solitary bird, for the individual the likelihood of becoming a victim is greatly decreased due to the higher availability of targets to choose from. A flock also benefits by being much more vigilant than an individual could be and is able to recognize threats much sooner and alert the other to flee the danger, overall increasing all birds likelihood of survival from predation.