Looks to me like some kind of rove beetle.
I wish your picture was a little bigger. Looks like it has transparent wings, can you see if it sometimes folds them up very small so that it appears wingless?
The size, shape, antenna, flying, and the super speed makes me think of a rove beetle. Plus, rove beetles would likely be found in rotting materials such as damp litter.
Rove beetles are good! They are predators, and hunt even smaller creepy crawlies such as thrips and other tiny things that eat rotting plant material. They would be a normal part of a compost ecosystem. Nothing to worry about. Just a sign that the chickens' litter is starting to compost.
Best - exop