[Physics FAQ] - [Copyright]

original by Blair P. Houghton 1992.

What is a Dippy Bird, and how is it used?

The Anatomy and Habits of a Dippy Bird

1.  The armature: the body of the bird is a straight tube attached to two bulbs, approximately the same size, one at either end.  The tube flows into the upper bulb, like the neck of a funnel, and extends almost to the bottom of the lower bulb, like the straw in a lemonade.

2.  The pivot: at about the middle of the tube is clamped a transverse bar that allows the apparatus to pivot on a stand (the legs).  The bar is bent very slightly concave dorsally, to unbalance the bird in the forward direction (thus discouraging dips to the rear).  The ends of the pivot have downward protrusions that hit stops on the stand placed so that the bird is free to rock when in a vertical position, but cannot quite rotate enough to be horizontal during a dip.

3.  The wick: the upper bulb is coated in fuzzy material, and has extended from it a beak made of or covered in the same material.

4.  The tail: this has no significant external features, except that it should not be insulated (skin-oil deposited on the bird's glass parts from handling will insulate it and can affect its operation).

5.  The guts: The bird is partially filled with a somewhat carefully measured amount of a fluid with suitable lack of viscosity and density and a low latent heat of evaporation (small d(energy)/d(mass), or "ld").  For water, ld is 2250 kJ/kg; for methylene chloride, ld is 406; for mercury, ld is a wondrous 281; ethyl alcohol has an ld of 880, more than twice that of methylene chloride.  Boiling point is not important here; evaporation and condensation take place on the surface of a liquid at any temperature.

6.  The frills: any hats, eyes, feathers, or liquid coloring have been added purely for entertainment value.  (An anecdote: as it stood pumping in the Arizona sun on my kitchen window sill for several days, the rich, Kool-Aid red of my bird's motorwater faded to a pale peach.  I have since retired him to the mantlepiece in the family room).

7.  The bird is operated by getting the head wet, taking care not to make it so wet that water drips down the tube.  (Water on the bottom bulb will reverse the thermodynamic processes.)  The first cycle will take somewhat longer than the following cycles.  If you can keep water where the bird can dip it, the bird will dip for as long as the ambient humidity remains favorable.

How does it really work?

Short answer: thermodynamics plus mechanics.

Medium answer (and essential clues): evaporative cooling on the outside; PV = nRT, evaporation/condensation, and gravity on the inside.

Long answer:

Initially the system is at equilibrium, with T equal in both chambers and PV/n in each compensating for the fluid levels.  Evaporation of water outside the head draws heat from inside it; the vapor inside condenses, reducing PV/RT.  This imbalances the pressures, so the vapor in the abdomen pushes down, which pushes fluid up the thorax, which reduces V in the head.  Since P is decreasing in the abdomen, evaporation occurs, increasing n, and drawing heat from outside the body.

The rising fluid raises the centre of mass above the pivot point; the hips are slightly concave dorsally, so the bird dips forward.  Tabs on the legs and the pivot maintain the angle at full dip, for drainage.  The amount of fluid is set so that at full dip the lower end of the tube is exposed to the vapor.  (The tube reaches almost to the bottom of the abdomen, like a straw in a lemonade, but flows into the head like the neck of a funnel.)  A bubble of vapor rises in the tube and fluid drains into the abdomen.

The rising bubble transfers heat to the head and the falling fluid releases gravitational potential energy as heat into the rising bubble and the abdomen.  The centre of mass drops below the pivot point and the bird bobs up.  The system is thus reset; it's not quite at equilibrium, but is close enough that the process can repeat this chain of events.

The beak acts as a wick, if allowed to dip into a reservoir of water, to keep the head wet, although it is not necessary for the bird to drink on every dip.

Is that all there is to know about dippy birds?

Of course not.  Research continues to unravel these unanswered questions about the amazing dippy bird:

1.  All of the energy gained by the rising fluid is returned to the system when the fluid drops; where does this energy go, in what proportions, and how does this affect the rate at which the bird operates?

2.  The heat that evaporates the water comes from both the surrounding air and the inside of the head; but, in what proportion?

3.  Exactly what should the fluid be? Methylene chloride is an excellent candidate, since it's listed in the documentation for recent birds sold by Edmund Scientific Corp. (trade named Happy Drinking Bird), and because its latent heat of evaporation (ld) is 406 kJ/kg, compared to 2250 kJ/kg for water (a 5.5:1 ratio of condensed methylene chloride to evaporated water, if all water-evaporating heat comes from inside the bird).  Ethanol, at 880 kJ/kG, is only half as efficient.  Mercury would likewise be a good prospective choice, having an ld of 281 kJ/kG (8:1!), but is expensive and dangerous, and its density would require careful redesign and greater quality control in the abdomen and pivot-stops to ensure proper operation at full dip; this does, however, indicate that the apparatus could be made in miniature, filled with mercury, and sold as a wildly successful yuppie desk toy (Consider the submission of this FAQ entry to be prior art for patent purposes).

4.  Does ambient temperature have an effect on operation aside from the increase in rate of evaporation of water?  I.e., if the temperature and humidity can be controlled independently such that the rate of evaporation can be kept constant, what effect does such a change in ambient temperature and humidity have on the operation of the bird?  Is the response transient, permanent, or composed of both?

Dippy Bird Tips

They have real trouble working at all in humid climates (since the evaporation of water from the head is lower for higher humidity), but can drive you bats in dry climates (aside from the constant hammering, it's hard to keep the water up to a level where the bird can get at it. . .).

If you handle your bird, clean the glass with alcohol or something; the oil from your hands has a high specific heat, which dampens the transfer of heat, and a low thermal conductivity, which slows the transfer of heat.  Once it's clean, grasp the bird only by the legs or the tube, which are not thermodynamically significant, or wear rubber gloves.

The hat is there for show; the dippy bird operates okay with or without it, even though the hat may reduce the area of evaporation slightly.  Ditto the feathers and the eyes.


Chemical data from K. Gieck, Engineering Formulas, 3d. Ed., McGraw-Hill, 1979, as translated by J. Walters, B.Sc.

I've also heard that Scientific American had an "Amateur Scientist" column on this technology a few years ago.