500 trees! Is this a commercial venture?
What size or kind of trees are they?
I used to have a mango, lychee and lamyai orchard with lots of trees....and I also kept about 50 chickens and some ducks free range amongst them.
I never had any problem with my chickens harming the trees. They were good for them..tbeir poop fertilised the ground and they ate pest insects. They used to roost in the trees.
Maybe it's the heat, lack of shade or lack of water, or even pests killing your young trees?
Seedlings stand a very poor chance in this climate if simply planred out in the dry season.
They need to be grown in pots, under cover for shade, and get regular irrigation, until they are large enough and sturdy enough to be planted out....which is best done in rainy season.
If you start killing your neighbours chickens you are going to stir up a lot of trouble.
Another poster suggested getting a dog. That is a very good idea. You could leave it to run about on your land and it would chase away the chickens and then they would be deterred from coming back.