Flocking for fun and profit

I was watching a bunch of birds the other day and I took note of their flocking behavior. I found it so intriguing that I decided to do some research. After reading all that I could find on the subject, I’m left with a sense of inadequacy with the theories. Everything I read indicated that the principles of flocking have to do with a sort of polarization - the birds are all aimed in the same general direction and take course corrections from their neighbors. Also, there’s a sort of collective homing mechanism that controls the fundamental direction of the flock. All sources agreed that there is no leader.

All that makes sense, when the flock is migrating, but it seems to fall apart when viewed with the behavior I was witnessing. Let me try to describe it. The birds seemed to be feeding. They started out in a tree. There were a lot of them; a hundred or more. As if a single unit, all of the birds dropped out of the tree and began to feed. Their flight from the tree was anything but polarized. As they left the tree they were fairly orderly, but in mid flight, they seemed chaotic, more like a swarm. However, when they touched down, they seemed to be reoriented in basically the same direction again. They fed for about 30 seconds, milling about at random on the ground, then as a single unit they ‘jumped’ back into the air. At first I thought that they were headed back to the tree, perhaps startled by something. There were two problems with this assumption: (1) There was no apparent source of ‘startlement’ and (2) they didn’t go to the tree. They just flocked over to a new feeding location. The route they took, by the way, seemed entirely random. As a group they darted this way and that, only to land in the new location as a group again. I watched a few minutes longer and again the flocked jumped to a new location. Finally, the entire flock jumped back into the tree. Through this entire episode, their timing seemed instantaneous. All of the birds seemed to decide that a particular feeding location was exhausted simultaneously. Their departure direction seemed predetermined. Frankly, it’s hard to see how they could possibly react that fast and that precisely even if they were following a leader… it’s even harder for me to fathom that this behavior is possible without a leader. Certainly a collective intelligence couldn’t process direction changes that fast. Also, in this kind of scenario, it seems unlikely that there’s any collective homing mechanism setting their course direction. However, I’ve been watching a lot over the last several weeks and it certainly doesn’t look like there is a specific leader either…

Note: My question is not about how flocks form or why - It’s about how flocks, as units, align to a specific goal. I know about boid theory. This explains how the flock is formed and why it is cohesive, but doesn’t explain how the flock knows where to go and when. In most of the interesting boids demonstrations I’ve seen, there’s a cheat added. One or more of the boids is given special directives that has the tendency to drive an element of randomness in the swarm direction (in other words, there’s either a leader or all of the boids have a common ‘mind’, giving them a common goal). True-to-life boids demonstrations tend to be uninteresting because the flock just moves in a straight line from one ‘side’ of the simulation environment to the other ‘side’.

Any flocking experts out there?

I thought you wanted to know how to apply “flock” to handcraft projects. Sorry!

Not a direct answer but some thoughts on it.


IMHO to some extent group flocking behavior looks “instantaneous” to you because birds are too small and their nervous systems and reflex responses are moving too fast for you to be aware of the micro-level feedback responses going on continuously. If you look at a of flock of vultures you might more clearly see the dynamics of the process.

Flocking behavior is, to some extent, a biological “voting system” wherein there is a cascade effect once a certain trigger point has been reached for food scavenging, flight from predators etc, which is buttressed by a fair amount of pre-wired neural pathway directed behavior.


Thanks for the link. I had not seen that one, however it really doesn’t answer my question any better… Perhaps it’s because I failed to ask it clearly.

I can see how this theory can easily explain what Bedau called “global-level flocking” (but what I would prefer to call macro-level flocking). What I don’t understand are the principles behind micro-level flocking.

In the macro-level flocking model, the vector for each bird (boid) is adjusted based on proximity, speed and direction of nearby birds (boids). This has the effect of subtle course changes because the current direction of the flock is sort of a rolling average of all the birds (boids).

In micro-level flocking, course changes are much more dramatic. The birds swoop, the flock darts left, right, up, and down. This strikes me as inconsistent with the notion that the flocking behavior is based on the the three simple rules in the boids model. Clearly there is some other, overriding goal seeking that is driving the flock. In fact, it makes sense that the basic rules are still in effect, which explains the sort of hysteresis that you see as the flock swoops about, but what is it that causes the entire flock to make a 90 degree turn, nearly instantaneously.
On the subject of instantaneity, you wrote:

This certainly occured to me and I’m sure that this is, in part true, however if you look at the kinds of turns that the flock makes, it’s still inconsistent. You would expect smooth gradual swooping to be the norm, but in fact, the flock exhibits sharp banking and dive bombing behavior inconsistent with the average vector theories. Even if you assume that the reaction times of the birds (boids) are quick enough to explain the quickness of the turns, it doesn’t explain how the average vector for the flock (essentially it’s collective intent) can change so quickly.
I think things like wind direction may play a big role in this micro-level flocking behavior. Wherever the food is, probably has some effect, as well. Maybe there is a leader, of sorts, and that leadership can be handed over very quickly, based on some simple rule(s). Perhaps there’s some ingrained requirement to ‘juke’ about, which might enhance the flock’s ability to locate food or improve it’s survival around predators… Or maybe I’m just full of crap and should just accept conventional wisdom…