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One of the most humane and effective deterrents I've seen is a water and noise system that's motion activated. This one is called The Scarecrow and you can see a video of it working (it's actually pretty funny) here:
Click ‘demonstrations' on the left side of the page when you get there.
There may be other brands or availability online so you might want to shop around.
As for the other things promoted in garden stores and online sites, including fake owls or other birds of prey, fake snakes and so on - they don't work. Birds are pretty smart and they quickly figure out what's a real threat and what's, in their mind, little more than a decoration.