There is a window ledge here above my porch door. Phoebe birds are great for catching bugs, but they are messy housekeepers. They crap on everything! I swept about 5 partial nests off this ledge before realizing the birds had more time for this than I do. CD 154s would stay on the ledge, but could easily fall off. CD 108s fit nicely and are discouraging the birds from nesting on this ledge. There are many other nesting sites around here for them to work with. Good use for my spare 108s.