A crowd simulation would be a terribly expensive idea for something as small as birds in your level, particles have much less overhead and are specifically designed to simulate things for level decoration, I have used them for birds before via a simple Flipbook animation. The Vehicle Game demo on Marketplace (free from Epic) also has a cool vertex-shader bird in it that’s part of a particle system. It’s a much more sensible way to go about it.
Learning the ins-and-outs of Cascade is a must IMO, it’s a very flexible tool if your good with material shaders and its next iteration (Niagra) should be even more so.