I have found that some years and some times of years are worse than others but there will always be birds of pray lurking around. They seem to figure out any schedule, I think the trick is to not have a schedule. But I have realized there will always be oppotunists around as you say, when you and the birds least expect them. I've gone to just letting my birds out all day everyday and they are smarter and faster for it. Lately we had a redtail trying to get em, they just flew circles around it, playing, and teasing it. It finally just landed on the roof right next to them, tuckered out.
I have a friend who also uses decoys, but he uses grazing geese decoys which he moves around the yard daily. They looked pretty cool too, like the real thing, canadian geese. He said they work! He got them from a sporting goods supplier. Here in the states he got them from Cabelas.
Good Luck
Carol