If that is how you fill it, the water when it fills the bowl blocks the opening so air can't get into the large chamber.....this prevents the water from flowing out because of the suction (no air to replace the space the water leaves...so creates and inward suction (sort of) and holds the rest of the water in place. As the chickens drink and the ware in the cup gets below the 'hole' that leads to the main chamber, a 'glug' of air goes in, and allows a 'glug' of water to come out...this continues until water in teh bowl is again above the hole into the main chamber....preventing more air going in, so no water can come out.
When you pour from a full gallon of water/juice/milk....it comes out in 'spurts' or 'glugs'...then sorta stops as air goes in, then more milk/water/juice comes out. It is almost like the bottle needs to breathe. What is really happening is as the liquid comes out, the space needs to be filled by something (or, the bottle needs to collapse/reduce space by the amount that came out), hence the sudden in-pouring of air to fill the space...once that happens, no suction holding liquid in, and more comes out & the process repeats until you either right the bottle so the liquid can't flow out...or you hold the bottle in such a way that liquid flows out bottom half of opening and air flows in top half of opening, and exchange is seamless. (like when the bottle is half full and you tip it only partially.)
I don't know if I explained this in a way that makes sense to you...here is a link:
https://physics.stackexchange.com/questions/151452/how-do-simple-bird-waterers-work