The Happy Bird Transpiration Toy

Today you will do a project with the "Happy Bird". Perhaps you have seen this "toy" on a shelf in an office or on a shelf behind a bar (the bartender wants you to behave like the bird!). The bird, once started, works something like a perpetual motion machine. The bird tilts forward, dips its beak in a glass of liquid, then stands back up. How can this bird keep moving with no input of food or drink (energy!)?

The bird is made of glass. If you look closely you can see that its abdomen is a bulb filled with a colored fluid. Fused into this bulb of fluid is a tube that forms the bird's axis. The head is another bulb. Although you cannot see inside the head, its structure is similar. The head and beak are covered with a kind of "fuzzy" flocking. The hat and tail feather are just decorations. The body pivots in a "hip joint" atop large legs and feet that stabilize the bird as it "drinks".

First Approach. If you try to start the bird by poking at it, you will be frustrated. It is good to be sure the bird's body is freely swinging in the pivot, but the bird will not start drinking on its own! It is not a simple pendulum; it must take something else to get it going, some continuous application of energy that your fingers cannot supply.

Second Approach. Hold the abdominal bulb in your fingers for a moment and observe what happens to the fluid inside. Is the fluid expanding or is the gas in the bulb expanding? While still watching and holding the bulb, you should be able to answer this question. Perhaps you can describe the properties of the fluid inside the bulb. These properties are essential to the function of the bird. If you think about this for very long, you will realize that you do not want to drop this bird on the floor! Treat it as a fragile object that it is; you probably noticed the safety warning on the box. Readers play it safe!

Now, if you let go of the abdominal bulb you should be able to see the bird dip to drink and stand back up. Observe closely. What makes the bird "tip"? What makes it stand back up?

I am sure you are disappointed that the bird does not go through another cycle...even after the fine start you gave it!

How this Second Approach worked. Perhaps you have thought about how the fluid went up the body in the first place and have decided that it is a pressure phenomenon. Does the Poiseuille equation come to mind? It should!

Flow = π r4 (8η)-1 ⋅ ΔΨp Δx-1

Clearly the value of pi (π), the radius of the body tube (r), the fluid viscosity (η), and the length of the body tube (x) do not change, so to get the fluid to go up the body tube, a pressure difference (ΔΨ) between the two ends was needed.

When you heated the abdominal bulb with your hand, you expanded the gas above the liquid (PV=nRT) in the abdominal bulb, but the brittle wall of the bulb simply converted the energy into increased pressure. The pressure pushed the fluid up the tube to increase the gas space. Compared to the low pressure in the head bulb, the increased pressure of the abdominal bulb provided the pressure difference. The fluid moved into the head bulb, the bird tipped, the pressure equilized as gas bubbled between the two ends along the now-horizontal body tube, and the bird stood back up.

So, we know now that it takes a pressure difference between the two ends of the bird to invoke the Poiseuille equation to cause flow along the body tube. We could do it with heat applied to the abdominal bulb as you did with your hands. Yet on the shelf behind the bar, there is no heat source for the abdominal bulb.

How else could you get a pressure difference between the two ends of the tube? If you cannot heat the abdominal bulb you could....

Yes! cool the head bulb. There is no ice under the hat of the bird in the bar, so how else could you do it? Notice the flocking on the surface of the head and beak!

Third Approach. Give your bird a "swirlie" in the cup of water and stand it back up. The water and flocking combine to increase the evaporative cooling from surfaces on the head thanks to the heat of fusion (heat of vaporization) of water. The cooling reduces the pressure in the head bulb (PV=nRT) and thus "sucks" the fluid up the body tube. Thus the movement of liquid in the tube is caused by low pressure at the top rather than high pressure at the bottom. Instead of being "pushed" up, the liquid is "pulled" up.

Your bird should now be tipping and drinking regularly. But this will obviously stop when the flocking on the head dries out and evaporative cooling can no longer provide the ΔΨ to drive the fluid movement in the body tube. However, you can replace the lost water if the water level in your cup is high enough. The bird's bill should go into the water and the flocking should restore the water on the head lost to evaporation. As long as the water is high enough in the cup, the bird should continue to operate. This is not a perpetual motion machine because someone has to replace the water in the cup!

Extensions. What environmental factors would acclerate the rate of dipping to drink? Since the rate of evaporation is the driving force, I hope you are thinking about ambient temperature, humidity, and wind!

This little bird has been an interesting toy, but what has it taught you about transpiration in plants? I hope you have observed some very interesting parallels, yet some important differences.

With a fuller-understanding of this model, I hope you will be able to assign botanical names to the abdominal bulb, the plant process equivalent to the heating you applied to the abdominal bulb, the body tube, the head bulb, and the flocking material on the head. If so, then you probably understand why we spent time playing with this interesting toy!