If you're talking about REALLY distant birds, the simple solution I use is to create a tube or two in Paint Shop Pro and use those in post. Because of the distance and consequent lack of detail, the shapes are very simple to create; a few of similar size but in different orientations, and you can easily use the random sequence of the tube to create a flock. Varying the gamma, size and position is easy too by placing them on a separaate layer which can be moved, sized and adjusted without affecting the underlying image.