I recently read an article in our local paper about the dangers of open-top posts. Many fences and signposts are made from PVC or metal pipe and if their tops are not covered, they become deathtraps to birds seeking to use them as nests. My husband and I have some metal fences, but fortunately, their tops are covered with a flat slab of metal. A small screen could just as easily be used–anything to keep a bird or a small animal from falling inside the post. For more info regarding the hazards of open-top posts to birds, check out the California Audubon website.