I’ve catalogued lichens on dozens of ridges, and their growth rate is painfully slow. The green scars you see are more about moisture pockets than bird paths. If a flock stops on a ridge, the lichens will only notice when the feathered traffic leaves behind a moisture bloom. Still, if you can line up a pattern of lichens with a known migratory corridor, it might just be a coincidence. Don’t mistake a frost line for a flight path.