The cups have little valves that the birds have to trigger to let the water out into the cups. They bump their beaks on the trigger when they go for the last bit of water at the bottom of the cup and that releases more water. Gravity is the perfect pressure for these waterers. Too much pressure and the birds won't be able to trigger the valve. It'll be too tight.
I've see most of those pics of feeders myself. I may try something just like one of those.





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