I built a contraption called Manitoba Fly Trap. A black swinging ball suspended in the middle of a pyramidal shape attracts the flies, which circle the ball then fly upward to the area that has plenty of daylight. I noticed that a type of sparrow was going in there and feasting on the flies. There is a trap up top but the flies never got that far as they were easy pickings for the birds. OK you don't have a trap. Go to your local mechanic and ask for some dirty engine oil. Wipe it on the chest, neck and legs, with the hair, don't rub it in. It's dirty but most effective. Before using it, leave it sit open for 24 hours. Be sure to wear old clothes. When flies are bad like this the horses need to be able to get into a dark shelter, a very dark shelter. The flies won't go in.