My blackberry and raspberry bushes are a fair distance from the house, and the squirrels have been able to make holes in my nets, and the birds have thanked them.
How does a drop line work?
Well, did a little research and the method is not quite what I thought- which is to string ropes covered with pitch (tar) on bushes and trees to catch birds.
Actually the usual practise was to smear the sticky stuff on the branches of the trees and “birdlime” was used instead of pitch. The birds would return to the trees at night and get stuck and the hunter would whack them over the head and take them to the pot. I think grouse, pheasant, turkeys and the like were hunted this way.
Birdlime can be made from holly bark or mistletoe or other stuff. A substitute from pine sap or the like would probably work, maybe even roofing tar. I saw a recipe for making ‘mouse trap’ glue from boiling down corn syrup that may work.
Probably outlawed though I found no authoritative sources.
Birds are easy but squirrels are tough- I assume you’re not in a rural enough area so squirrel pie or gravy isn’t feasible.
You could relocate the raspberries closer to the house and/or let so many grow that the squirrels can’t eat them all. Get a dog that doesn’t like squirrels, cut back the woods.
Squirrels are difficult!