That used to be the case.
In 1972, an amendment added 32 families of birds to the treaty, including owls, hawks and eagles, bringing protection to 1,026 bird species, almost every native bird on the continent or any bird that strays into the U.S.
Included in the treaty are Canada, the US...
That pattern is exactly what I've witnessed on many occasions. Every time a hawk tries to test them. I haven't had a hawk loss since I've kept at least one rooster with each flock.
In the absence of the ability to keep roosters, opt for the more skittish, wary breeds like Anconas, Leghorns...