Why isn’t a drinking bird perpetual motion?
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If you have liquid water in a room that’s below 100% relative humidity, it wants to evaporate until the room is fully saturated with water vapor. When a process wants to happen, you can sometimes put a minor obstacle in its way so it’ll overcome the obstacle and do useful work for you. That’s the basic idea of how hydroelectric dams work.
It’s hard to turn heat into motion because motion is more organized than heat, and everything in the universe wants to get less organized. But a room full of water vapor is less organized than a glass of water in a dry room by a larger margin than a warm room is less organized than a cooler room and a moving bird. So when the drinking bird runs, the room containing the bird and the water gets less organized, even if some parts of it (the bird’s motion) have get more organized in the process.
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