Roosters can be effective against hawks, but not all or all of the time. Bantams, silkies and Polish probably close to never. Immature (< 8 months) roosters never. Roosters in molt where tail and body feathers being replaced no. Rooster in poor health or of low rank no. Rooster in open without cover while hawk is above no.We are definitely learning as we go along with our chickens. We never grew up around them or anything so I did research before I got them in August 2018, trying to learn as much as I could about them and more research as new problems arise. I don't know a whole lot about roosters, I didn't even want him lol. He was supposed to be a hen when I first purchased my chicks at the beginning. I just assumed if he was trying to attack humans that he must REALLY love his hens, therefore was a good protector for them. Guess not.
Hawk on ground better. Hawk on ground where rooster can attack from cover better yet. Rooster that is a daddy with developing eggs or chicks nearby will be better yet. Game rooster best.
I have been watching how chickens interact with raptors for a while. Their patterns in variables that protect outcomes. Most people keep chickens in mowed areas with inadequate cover. The roosters are immature, represent breeds with limited physical ability, or in molt. Rooster does not win every time, but most people are not even aware of encounter unless the rooster looses.
I have as many hawks as anyone, yet have very few losses to hawks. Make so the chickens have in environment that helps the chickens in such encounters. And no bantams or all hen flocks in free-range settings with lots of chicken eating hawks.