Is there a way to achieve the look of a distant flock or shoal without full simulation? Although simulation models like Boids give good results for bird flocks or fish shoals on a small scale, simulating every single member in real time becomes unrealistic for huge numbers. Is there a way I can model a flock in the distance where only the density of birds is visible? I'd like to have that flowing, changing density gradient with a much smaller number of variables to process.

I've tried using a much smaller population and displaying each boid as a blurred area with Gaussian density so that as they overlap the density rises and falls through their interaction. This is reasonably cheap but it never leads to sharp changes in density, either spatially or temporally, which makes it look too uniform.

Is there any other way of getting away with a much smaller number of individuals? Or is the only way to get realistic results to prerender?
One way that might work is to have boids but render each boid as a group of birds.  This way the simulation is still simple but the rendering makes it seem complex.  You might even allow a little drift per bird in the boid to make it look so obviously rigid.