Old German Traditions
       My mother-in-law always kept a real bird's nests in at least one of her Christmas trees, usually the live tree on her back screened-in-porch. She believed that it would bring her money in the following year. She wasn't really superstitious; she just liked to pretend to be superstitious. I know, it sounds strange but many old customs seem strange. As long as those customs are harmless, I think that it is fun to keep them. Bird nests in a Christmas tree are kind of like fortune cookies. Everybody hopes that they bring good luck but, we all know that it makes no difference in the end.  
      The first year that my husband and I were married, we found an abandoned nest in a tree during a fall walk. He climbed that tree to retrieve it for our first Christmas holiday decorating party. It was very romantic! So, every year I put some type of bird's nest in our Christmas tree for luck. We are still poor, though, even after all of our annual efforts to add one of these lovely nests to our Christmas displays. 
      If you are going to add a real nest to your tree, make sure that it has been abandoned. If you collect the nest during a warm month, seal it for a few weeks inside a airtight plastic bag along with a mothball or two. This will kill any insect eggs that happen to be inside the nest. Moving a nest into your home during a warm month will cause those little pesky critters to hatch inside your house! If you find the nest in winter, after the first frost, the insects should be dead.

About birds' nesting in Christmas trees:
I Have a Real Bird’s Nest in My Christmas Tree