I think some birds are actually pretty smart and can learn by imitating other birds, so it's a cultural thing. All it takes is one bird to discover it.

Take a look at this video: "Crow uses a vending machine". It was an experiment where they trained crows to collect coins and put them into a vending machine for a few grains.

https://www.youtube.com/watch?v=1qSsVBPh9Lo