Answer: You could use some scare tactics, such as hanging milar (shiny) ribbons or old CD's from fishing lines, but they might look unattractive near your window boxes. If they were my planter boxes, I'd line them with moss and then put bird netting over the moss. Birdnetting has one-quarter to one-half inch holes which will make it more difficult for the birds to steal the moss. If you pull it tight over both sides of the moss, you may not even notice the netting is there. Or, you might try using coconut coir rather than moss. It is more tightly bound and of a harder material that birds might not find quite as attractive.
Hope this information helps!
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