From wikipedia http://en.wikipedia.org/wiki/Chickenhawk_(bird) In the US, a Chickenhawk or Chicken Hawk is an unofficial designation for three species of North American hawks: the Cooper's Hawk, the Sharp-shinned Hawk and the Red-tailed Hawk. The term Chicken Hawk, however, is inappropriate. Although Cooper's and Sharp-shinned Hawks may attack other birds, chickens do not make up a significant part of their diet; Red-tailed Hawks have varied diets, though they too would be unlikely to attack poultry. Historically, misinterpretation of the name "Chicken Hawk" has labelled these birds as pests, hence justifying their slaughter. Officially, per the American Ornithologists' Union's list of bird names, the term has become obsolete as applied to birds, but still enjoys widespread colloquial use in rural areas where any of the three species has been seen as a threat to small outdoor animals kept as pets or livestock, especially chickens. While the term is still widely used by those who keep such animals, it is too ambiguous to be of any scholarly usefulness, especially since the meaning of hawk differs between America and Europe; thus, the term's propriety (or lack thereof) depends entirely upon context. Farmers tilling under their crops will have swarms of hawks hopping around on the ground behind the tractors grabbing the mice that have been displaced by the discs. These hawks on the ground look like chickens running around, hence the name. This is similar to calling a kestrel a sparrow hawk and a merlin a pigeon hawk.

It really means just any hawk that birds are afraid of. I've had coopers hawks fly over our poultry and you've never seen such action from fat lazy chickens. They dive for cover, run and fly to the coop or bushes, anything to hide from those hawks. And a few times, those hawks have taken a dive or two at the younger stock. They don't react that way to all hawks, though. There is an underwing pattern that chickens, or any prey animal, seems to be born with knowing it's trouble! They never reacted that way to red-tail.ed hawks. But they did to coopers and harris hawks. Lo and behold, underneath, they are marked similar to each other. I think it's the dark tail bands that they react to. Chickens know!