Who is leading the group? What’s their goal in making the formation? Are they hunting together?
The birds have, as far I know, no real leader but follow a simple rule when flying. Every bird position itself an exact distance to the surrounding birds. This means if one bird is moving more to i.e. the left will the neighboring bird also move to reestablish the exact distance again, and this make the next bird move and so on, all out to the last bird.
The reason for any animal to move in groups is because if a predator is wanting to hunt, is the chance of one particular animal being the victim, very low compared to if it was alone. But also when hundreds of animals, all are keeping an eye out, is the chance of spotting the predator way better than if only one animal had t look out for itself.
For little songbirds like that there is no permanent leader, they’re just naturally gregarious as a predator defense. It’s hard to chase down a single bird in a whirling tornado of identical birds, and predators get bamboozled.
They’re not hunting, most of their diet is seeds and small bugs they grab off the ground.
How they actually manage to swirl around in a big cloud without colliding has been the subject of a lot of research and modeling over the years. Each bird is eyeballing a few others in flight and adjusting to maintain position compared to them.
How they decide which way to whip the whole cloud is a more democratic process, with individuals near the edges picking a direction and the rest following whichever direction has more birds.
Sometimes you’ll see a particularly indecisive flock buzz about in place for a while because they can’t decide which way they want to go, or leave a few stubborn stragglers that don’t feel like going that way yet.
We think that birds which flock, like fish that school, do it because of the “safety in numbers” idea. Predators have trouble targeting a specific individual when the mass moves around like that.
Why and how flocks and schools behave like the do is a tougher question to answer. This sort of group action is called “emergent behavior”, which is a pair of big words saying that the group acts like a single individual for some odd reasons we don’t quite understand.
Somehow, the group makes decisions and each individual responds in kind to that group decision. A controlled chaos of sorts.
Really quite beautiful to watch but the how of it is not something we humans can agree on. They do it somehow. Humans on a crowded street do the same thing.
There are a lot of different reasons why birds flock together, but the main reason why they fly in tight formations is for protection.
An animal that is in a tight flock is harder to prey upon than one that is alone, both because there are a lot of other targets and because it is hard to track a single target amid the group.
And no one bird is leading the flock. Instead each of them is responding to each other and the air around them and all making the same majustments based on the same basic mental rules.