I am a falconer (a person who uses a trained bird of prey to hunt wild game) as well as a chicken owner. American Kestrels are too small to take down a chicken, so cross that off of the list. Your larger falcon species are aerial attack artist, so having them attack a chicken on the ground is more than likely not the case. Coopers hawks attack birds and can catch them on the ground, but a full grown chicken is quite the feat for a coopers hawk, so once again more than likely out of the question. You said that you live in Maryland. Check out the North American Goshawk. They are an accipter, larger than the coopers hawk, and when in adult plumage are grey and white. They are the bird and stunning to see. You face heavy fines if you kill this bird. I know its upsetting when a chicken is attacked and killed, I would lose my mind if mine were attacked and killed, but I live in the city and therefore they are not allowed to free range. Just remember that when you have a freen range flock that predators will take advantage of them. Its the vicious circle of life. Please let me know if it looks like a Goshawk. Hope this helps.