We were never making an exact comparison though. The point you've been arguing against (unless I misunderstand) is that creatures such as birds and fish have some special hardcoded behavior that the rest of us don't. I don't think spacing the humans out a bit more is going to solve the problem here; the aircraft example makes that point nicely.

In fact even your objection about visible gaps serves as a case in point. A flock of hundreds of thousands of starlings somehow avoids such dangerous densities even in the face of transient "pressure" waves traveling through it. Meanwhile human groups appear to willingly form such densities without thinking twice. Sounds like a hardcoded behavior to me.

If you really want an example with more density then consider schools of fish. Of course unlike birds it's not so easy for most people to observe fish. I don't actually know for certain that they never collide or get injured by being crushed. I seriously doubt it though.