I use fully adult well-feathered standard sized roosters in a measurable way to protect flock. First the activity is risky and resources in the environment such as cover are extremely important to the roosters success and likelihood to take such risk. The roosters do invest more in observing their surroundings. Mine are particularly effective against one class of predators, hawks, and the species of hawk where I really need roosters to defend against is the Coopers Hawk which goes after smaller birds in the flock rather than the rooster. They are not effective against animals like foxes, coyotes or dogs unless the predator is naive or has an impairment of some sort. I have seen roosters prevail against such but it is as a general rule a consumptive use of roosters when they do engage such predators physically. My roosters will run a distraction to get predator to follow them as offspring hunker down and slink to cover. My roosters then then fly and quit well to avoid being captured at last moment. Most chickens cannot fly so well. Also my roosters generally are not defending hens, rather they are defending their reproductive investment which means offspring and the protection is particularly evident when group is small like a harem and more likely to the roosters offspring. Other predators and at night generally rooster is much less likely to fair well. The roosters are the most vociferous in producing alarm calls that activate other anti-predator mechanism such as poultry guardian dogs and myself.
There is considerable variation as a function of breed and general health. Bantams are too small and those I have observed do not seem to have the appropriate behavior repertoire needed. Silkies and Polish have visual impairments and the former has serious physical limitations stemming from feathering. Birds with bad eyes or sick will not be up to the task. Low ranking birds are more concerned with self preservation.
Roosters are a tool in your predator management toolbox and should not be looked at as your only or even most important measure. Always think how to integrate methods and where practical present the predators with layers that not only make visits unpleasant or unlikely to get a meal, they also advertise they are entering a zone that is better avoided.
