Translation:Shulchan Aruch/Choshen Mishpat/213

Paragraph 1- If one sells the fruits of a birds-nest or the fruits of a beehive to another, he would acquire and it is not considered an item that is not yet in existence because he is not selling the birds that are born or the honey that will end up in the nest. Rather, he is selling the nest for its fruits or the beehive for its honey. It is like he is selling a water channel to another, in which case the recipient will benefit from that which is trapped inside. So too here, he is transferring this nest for its fruits just like one who sells a tree for its fruits.

Paragraph 2- The owner of the nest would not acquire the eggs and chicks themselves that are in the nest so long as they have not flown. This is an institution by the Rabbis that was made because of the prohibition of taking the mother bird from the baby birds. Thus, if one wants to transfer these chicks or eggs to another, he must hit the nest until the mother flies off and then lift these chicks or eggs from the ground. He would then transfer them to the recipient using one of the methods of kinyan for movable items.