I am a great fan of the Owl feather flies. The feathers of an owl are designed in a way to muff the sound of the approaching predator. They achieve this by very fine, velvety structure. This structure lends itself very well to fly dressing applications.
The fine structure shows the most in the tiniest flies. Here are a couple #20 owl feather flies, tied using Hanák 130BL hooks. They are displayed on an actual Eagle Owl feather. The feather has laid a while on my fly dressing table and I am afraid it got a tad dusty; the structure is still clearly visible though.
This pattern is best saved for grayling - the Lady of the Stream, being a creature of gentle disposition, shows great appreciation of fine and delicate flies. At the same time it has no pointy teeth to ruin your precious imitation. Brown trout, ugly beasts they are, will ruin the herl body in no time. They are better served by more robust patterns.