For its flight:
* As dark and silent as a shadow.
* As swift and elusive as a whisper in the night.
* Like a feathered dart in the twilight.
* As graceful as a black ribbon against the moon.
* Like a phantom gliding through the air.
For its appearance:
* As black as midnight.
* Like a furry, winged creature of the night.
* As intricate and delicate as lace.
* Like a living, breathing shadow.
* As mysterious and enigmatic as a sorcerer's cloak.
For its behavior:
* As keen and perceptive as a hunter.
* Like a master of the night.
* As determined and relentless as a predator.
* As curious and inquisitive as a tiny explorer.
* Like a creature of instinct and intuition.
Choose the simile that best fits the context you're writing in!