A question that is often asked is why birds flock. And why do some of them form mixed flocks?
It is certain that birds flock for evolutionarily sound reasons: to avoid predation and to find and capture prey.
By flocking, birds have a better chance to avoid predators than when they are by themselves. First, with more birds together, there is a higher level of alertness, as individuals are likely to be looking in different directions, and not all of them will have their heads down at the same time. In fact, it has been found that birds spend more time feeding and less time looking around when in flocks. Some shorebirds that feed by constantly probing the substrate, for example godwits, may flock with other species that forage with heads up, for example curlews, for the added vigilance.
Second, when birds fly in flocks, they make it more difficult for aerial predators such as falcons to catch them, because the falcon has to concentrate on a single bird, while a group promotes confusion. Furthermore, a bird in a flock of 20 has only a one in 20 chance of being caught, whereas a bird by itself has a statistically more serious problem. When a Bald Eagle flies over a flock of coots, they scatter in all directions.
Third, birds in flocks can actually intimidate predators. Starlings form a swirling superentity and dive on any hawk that approaches them; the tactic works quite well.
There are other advantages. Birds may discover food by associating in groups. Vultures and gulls roost in flocks, then spread out to feed while keeping a distant eye on one another. A vulture that drops to the ground is at the center of a contracting circle of birds heading in its direction. Gulls and terns have been attracted from boats by a handkerchief thrown in the air, simulating a bird dropping to the water.
Birds may also flock to capture prey more effectively. Pelicans in a line drive fish in front of them. Auklets surround a herring school and control its movements just as border collies herd a flock of sheep (why don’t falcons do this with shorebirds?).
Presumably birds form mixed-species flocks for the same reasons. If a flock of 100 is more effective than one half its size, then it makes sense for 50 blackbirds and 50 starlings to forage and fly together. For the most part, birds of similar size and habits flock together, so you’re unlikely to see a mixed flock of murres and juncos.
One of the most commonly seen mixed flocks is a winter feeding flock. In this area, it usually contains chickadees of one or more species, often Red-breasted Nuthatches, and sometimes Golden-crowned and/or Ruby-crowned Kinglets. The flock may be joined by a Downy Woodpecker or Brown Creeper or Townsend’s Warbler or Hutton’s Vireo. By moving through the woods together, these birds may help one another spot particularly good feeding areas, and they are surely more alert to predators as a group than if they were foraging individually.
I wonder if it’s possible to explain any of our own social behavior by this reasoning. Or can we explain bird behavior by what we know of our own? Perhaps some birds flock just to check out members of the opposite sex for the next breeding season.