I have some pigeons mixed in with a variety of chickens. They are tolerated by many, less so by some. Those "intolerant" ones will occasionally chase the pigeons when they land on the ground or attempt to bathe in a water dish. The pigeon usually just flits away a bit and really seems unconcerned. They are not afraid of the chickens. My pigeon nest boxes definately are up high as I have the noseiest chickens that are always sticking their faces in places they don't really belong. I would be very concerned that an active pigeon nest, either eggs our young squabs, would fall victim to the chickens during a period when the adults were away. I've also been concerned about a young bird leaving the nest and ending up on the floor unable to get away from the large birds. Once this last batch of squabs leaves the nest I doubt I will allow the birds to breed amongst the chickens for the above reasons.