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.
Though is one particular brand, there may be others and of course other sources of sales, so shop around for a bargain.
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 little more than a decoration.
There is a device on the market that is a huge success and is harmless to the animals you’re trying to keep out – it’s called the “Scarecrow” (made by Contech). This is a motion sensor sprinkler system that also works to keep all sorts of critters out of gardens and yards.
Here’s a look at the device in action (no animals are harmed)
Here’s one to give you an idea http://scraminal.com