So I've been doing research to try to find out they are, (I have a lot of free time) and I think they might be black flies. Here is what I found out about them: their bite can inject toxins which could kill your chickens, they bite chickens in the first place because they need the nourishment for their developing eggs, your chickens could bleed a lot after they are bitten, they could also suffocate your chickens, by getting into their airways, and they could give your chickens disease. They are most active in the day especially, after dawn, and before dusk. I hope you do not have them but if you do you could get some fly powder, or you could place fans in certain areas the chickens go most, because the flies do not like moving air. Good luck, hope this will help!