I made a simple bird waterer made from two 1.25L PET bottles. Let's call these two bottles the reservoir and the tray.
The tray is a bottle with the top cut off to leave only the bottom third. The location of the horizontal slice really only matters to help stabilise the reservoir which will be inverted and placed into the tray. A window is cut into the tray to allow a bird to duck its head in and take a drink.
The reservoir is a whole bottle (compressed in the depicted image for formatting) with the lid removed and a small hole drilled a few cm's up (when inverted) from the neck. The location of this hole is important as it is a negative feedback loop to control the water level in the tray.
Fill the reservoir with water. Invert it and sit it in the tray. The water will drain from the reservoir through the neck until the tray is full of water and no more. As water is taken from the tray, more water drains into the tank to fill it. Once the tray is full, the water stops draining and remains held in the reservoir.
See a video of it here:
The way I thought it would work is the air hole allows air to leak into the reservoir which keeps the air pressure the same as the outside. Water flows out through the neck since the air pressures are matched outside and inside the reservoir. When the water level reaches the air hole, the hole is sealed and the 'vacuum' in the top of the reservoir increases until the air pressure pushing down on the water sitting in the tray prevents the escape of any more water.
This is not what happens though. The water stops rising before it reaches the air hole. You can see this in the video - I've annotated a blue box where air hole is.
So the question is - how does this really work?
I'm thinking it is to do with air pressure but also possibly surface tension. If the bored air hole was too large, intuitively the water would just fall out no matter what. Why though if the air pressure in the reservoir is lower than outside, doesn't the air continue to push through the air hole until it is sealed?