My opinion on the plastic owls is described in pix below.
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I have no shortage of hawks in my area. I keep my chickens safe by having a pretty secure run area. When I am outdoors, I will let them free-range in the open yard.
Another thing that I noticed, and took note of,,,,, The hawks seemed to come/fly by on a certain daily time schedule. It was not exact, but generally similar times. Then they moved on further on their way.
Only thing that I read as successful to keep hawks away,, are these funny looking kites, that are mounted on tall poles. (tethered)
Those kites also scare the chickens even more. Just cant win,

If someone has property, that kite can be placed out of chickens' sight, but visible to incoming hawks,,,,,, than it should be fine.
I do have a Plastic owl,, that I place as décor on my patio.