Trustworthy dogs are a great deterrent, if they are near the chickens. Hawks are not in the business of getting killed by a predator (a dog) while eating their meal on the ground, and so most hawks will avoid making a kill where a potential predator is. Unless you have chicks or small bantams, north american hawk species cannot fly off with a full grown large fowl chicken, so if there is a dog around they are less likely to try for a chicken. The main problem are young, inexperienced, green-behind-the-ears hawks that don't really know what they are getting themselves into, and might try to take prey that they cannot handle.