I have 3 new chicks & there is a Steller's jay stalking them. Very bold & clever. When they're getting fresh air, even though they're in cage, they're not safe - jay got in the converted dog cage; luckily chicks got out same cage & were safe. Now I don't let them out of sight for more than 30 seconds when they're in fresh air. I still have to chase him away. I may get a water-bazooka or pea-shooter.
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IF you’re out with them, keep handy a few Gatorade/similar bottles full or coins & rocks.
Try to be stealth about where you’re at in relation to the chickens, and when you see the jay, aim for it. Don’t worry about actually hitting it, it’ll see the missile coming. The point is a loud noise coming from “nowhere” when it contemplated hunting your birds.
Over time, jays can be trained away from a coop with a variety of annoyances. Noisemakers, Air/water blasters, sudden flashing lights (I’m pondering mounting one on my coop’s corner when it’s built, as the property has jays and I have experience)…
At my old place, I actually rigged a goofy little twine “zip line” with a plastic kite. I sat inside with the release run through the window, and when a tenacious bugger came sneaking around, a let it rip.
My girls zipped into their coop, jay looked like it had the scare of its life and never came back, and a minute later, I was outside pulling the kite back into place while my girls were singing about their fresh layer of scratch.