How do they avoid crashing into each other?
I’m not talking about a bunch of geese flying in formation, I mean a huge flock of say, sparrows swooping around. Cecil answered a similar question with gnats, but I find it hard to believe that a flock of hundreds of birds could orient themselves around a point particularly if they’re migrating.
Birds don’t really fly all that close together most of the time, and I have seen them collide once in a while. (They do recover well!) It’s the same thing as people moving around in a crowd.
There are quite a few flocking simulations around; the original is at http://www.red3d.com/cwr/boids/ and links to papers on various movement behaviours. The rules he used are, IIRC, 1)Keep moving, 2)Stay close to your buddies, and 3)Not too close!
Another thing about recovery after small collisions is that flapping-winged flying things are quite robust and correctable when compared to our flying contraptions. If two planes even nudge each other, it’s really serious. But insects can fly full speed into a wall, bounce off, and keep right on going. Sometimes you see researchers trying to figure out how to make artificial flapping wings due to that advantage, but for the most part they just can’t get it right. Birds also know practically to the cm where they are (ever see sparrows race through the trees?), and wouldn’t bump often anyways.
Or, better yet, fly through a chain-link fence without slowing down!
However, birds have hollow bones (which is one reason they are able to fly), so they have to avoid collisions. That’s why when they see a predator, they arise and fly as a flock, for the predator will not risk bumping into one, which could be a fatal collision. So I don’t believe they do bump into each other occasionally. Once may be once too often.
Certainly birds are relatively fragile, and a head-on at full speed would do bad things, but the collisions I mean are like those little bumps and pushes you get while walking in a crowd.
Hawks and eagles, of course, handle collisions pretty well as long as they are the ones causing the hit.