If you're finding bird nests on your window sills, there are a few things you can do to discourage them from nesting there. The best way to keep birds from nesting on your window sills is to make the area less attractive to them. You can do this by removing any potential nesting materials, such as twigs, leaves, and feathers. You can also make the area less inviting by covering it with netting or chicken wire. If you have plants near your windows, trim back any branches that might provide perches for birds. Finally, keep your windows clean so that birds don't mistake them for another surface on which to build their nests.