Squatter Rights

I am not sure when our next door neighbors placed two birdhouses on the walnut tree in our backyard. One morning, I looked up at the magnificent trunk and saw these small structures attached about 30' above ground. Granted, this walnut tree shades part of our neighbor's backyard, but the tree is on our side of a 12' fence. I have yet to say anything about this to our neighbors, a lovely young couple, when we give each other quick waves and get into brief chats about our gardens. I really do not mind that the birdhouses are there - I am just a little perplexed on what squatter rights pertain to birdhouses.