That's interesting, haven't seen these before.I use these....
Cup is always full...the yellow things is a float, not a trigger. So far so good. I do not use it in freezing temps though..
Linky
I assume that as the water level in the cup drops, the float drops and a valve opens to release more water from the vessel.
Wonders if the chooks pushed down on the float repeatedly, even when the cup is full, the cup might overflow?