I've had a motion detector sprinkler device called a Scarecrow (no kidding) for years. It is the best thing I've found for keeping birds and squirrels off my lychee tree. It even 'sees' at night so raccoons, rats and mice are also sprayed. My tree is about half the size of yours, though, so you'd expect to harvest the nuts from the lower part of the trees and let the birds have the ones up top, I guess. It sprays about 35ft. and up to 20ft high, depending how you stake it into the ground.
They're available on Amazon.
