I've always wondered about that. I know scientists have proven that the fear of the silhouette of a hawk shape is innate in chicks.
http://www.jstor.org/discover/10.23...2&uid=70&uid=4&uid=3739256&sid=21103034682053
I fly my parrots outside, so have read a lot on hawk behavior and have consulted a lot with a friend of mine, an avian vet who specializes in falcons and birds of prey.
As far as I know, none of my chickens (oldest 8 months) have had any close encounters with hawks, but my parrots most certainly have. The parrots are housed outside 24/7 most of the year (inside for the next few days with this nasty cold front). The parrots alert when a vulture flies overhead although they have calmed down a lot in the 18 months they have lived in Texas and been exposed to vultures. In the past few days, it seems the parrots aren't even alerting for the vultures anymore.
Next door are a large flock of older chickens, a couple of peacocks and it sounds like a goose or two.
My chickens will react to alarm calls from my parrots, the peacocks, goose and the chickens next door, even my dogs although I'm not sure they are alerting to the dogs alarm calls or they are more worried that the dogs will attack them. The other day the free ranging flock were with me, foraging in an open area when the geese next door started yelling. My young flock ran very quickly to the fence line, which has overhanging trees. Another time, my neighbor called to say she had seen a fox on her property. My chickens were very upset, even the deer were upset. I'm happy to see that all the prey animals seem to alert off each other.
So far, touch wood, I haven't lost any to predation. I hope my roosters will do their job and protect the flock, which is why I have them. They've had some safe practices of protecting the flock from big birds when my parrots have gone to ground near the chickens and the cockerels have attacked and chased them off--good safe practices so when something really dangerous arrives, they won't hesitate and will just attack. Hawks in general won't risk being attacked on the ground when they are compromised by holding struggling prey, so they'll give up. Hawks have a ratchet system around the tendons of their legs so that once closed, it takes no effort to keep their feet clenched. It is a two step process to release their grip, they first have to open the tendon and then it can slide. When they are in a panic, often they can't release their grip, so they tend to avoid being down on the ground grabbing something too big to fly off with when they could be attacked.
Mine don't respond to hawks. They respond to crows! And during winter, we have tons of them. The neighbor's chickens come down here. Day before last, the stray roo sounded the alarm when a HUGE hawk flew low. My chickens looked shocked.."What the heck was that??".