It is either a young red tailed hawk or a red shouldered hawk. A red tail will usually have a band of color across a mostly white chest, but one of the defining features is that when he is flying he will have black "shoulders" like in this picture:
A red shouldered hawk does not have those black patches:
There are other differences but that is probably the most straightforward way to tell. Red Tailed hawks -can- take chickens but generally their prey is rabbits and squirrels (think mammals) but red shouldered hawks do eat a lot of birds. Red shouldered hawks only weigh a couple of pounds or less so if you have standard sized chickens and you think this hawk is getting them then I would suspect its a Red Tail. I would suggest bird netting over your guys.

A red shouldered hawk does not have those black patches:

There are other differences but that is probably the most straightforward way to tell. Red Tailed hawks -can- take chickens but generally their prey is rabbits and squirrels (think mammals) but red shouldered hawks do eat a lot of birds. Red shouldered hawks only weigh a couple of pounds or less so if you have standard sized chickens and you think this hawk is getting them then I would suspect its a Red Tail. I would suggest bird netting over your guys.