I had 5 chineese geese last year that grew up with my chickens. They twarted numourous attacks from Cooper's Hawks. For about a week straight last fall when the hawks were moving through I personally observed probably 3 dive-bomb attacks a day. There's a slight dip right infront of the coop door, so I could never see what exactly happened in the last 2 feet before the hawk got to the birds but every time there was an aweful comotion from the geese and the hawk always left empty handed.
The geese NEVER showed any aggression towards us or the chickens, so it was great. However, the had angel wing and once they started to moult they kept busting blood feathers and the chickens would be all over them.... after awhile the geese started to get cranky towards the chickens and I figured it was time for them to go.
They did a great job at deterring hawks... and although they were pets and a part of me does miss them, I have to admit I do enjoy the peace and quiet. You couldn't get within 25' of the run or coop without them all honking their heads off. Couldn't even think of carrying on a conversation with someone while standing in the barn or by the run.
I'm going to try to get one tom this year and see how that goes...