This is clever. But in a way, it's also shifting the environmental impact from these bottle caps. I can't imagine under which conditions the birds' may be a problem (e.g. getting too much food). Maybe someone with better knowledge on these species can tell if there could be a downside (no matter how improbable, just the risk of X). Such risk may include the interaction through second or third organisms (e.g. lack of seed spread, abundance of parasitic prokaryotr, etc.)
Biological question aside, anyway... This is so cool! I'd like to try building one myself ;)