My neighbour was going on holiday for a few weeks and asked me to check in on his bird's water supply each day. I figured I didn't need to do it every day until I saw his watering apparatus.
Oh dear. Not to worry, as a kid on the farm we had chooks that needed water and I remembered it was type of like an upside down bucket that somehow didn't let all the water out in one go but drip fed it out as required. A project was required to remedy this poor birds water quality and after some experimentation I had the solution. The kids readily agreed when they realised we would have to consume 2L of fizzy drink in order to implement it. So, start by acquiring two PET bottles. We used two 1.25L bottles.
Lop off the top of the first bottle about a third of the way down and cut out a window for the bird to stick its head in and drink water from.
The second bottle will sit inverted in the first. Unscrew the cap and discard it. Drill a hole such that it sits in line with the top water mark of the first bottles drinking window.
Fill the bottle with water, tip it upside down and sit it in the first bottle. The water will drain out from the neck of the bottle, while air is sucked in through your drilled air hole. The water will rise until the air hole is blocked and no more air can enter the sealed chamber. Once the air hole is blocked, no more water leaves the inner tank. If you have aligned your drinking window and air hole correctly, then the water level will be at just right level for your bird to drink from. If the water level drops, the air hole is opened and water drains into the tank again. If you drilled the air hole too high, the water won't rise enough to close the air hole, so your bottle will just continue to drain until it is empty in a minute or so. Drink another litre of soft drink and drill your hole lower down this time. I suppose the way it works (open to a real physics instruction here) is the air pressure inside the sealed bottle balances the gravitational pull that wants the water to drain out. The air hole acts as a negative feedback loop, allowing the bottle to take in air and leak water.
Once the water in the drinking pool rises to cover the air hole, then the air pressure in the sealed bottle halts the release of any more water.
Update: After closer examination, the hole is never covered by the rising water. Surface tension seals the hole. See the second video
Make a video of your creation.
I'm pleased to report the bird hasn't expired from thirst yet and the water reservoir will only need a refill about once a month at observed rates of consumption.
Think harder about how this works. While making the first video my speculation of how the physics operated didn't match what I was seeing. I asked a question on StackExchange and made this second video with some observations of what is happening.