Brilliant. Made me think that maybe you could do a drinking glass that has contents that move with gravity? I guess that would be back into the arena of hand-held props/animation issues.
You're absolutely right, sir. For that kind of prop, the held one would be the way to go, I think. One thing to have into account when modelling the animation of the liquid surface so it's always normal to the gravity force, it's that, assuming a glass of circular section, when the glass is tilted the liquid surface will become an ellipse, with an eccentricity larger as the angle of inclination of the containing glass grows. It can be done, by using a series of bones associated to the vertices around the surface of the liquid.
However, there's a caveat to this approach, should it be implemented: the illusion of a correct simulation will be ruined if the puppet holding the glass concurrently performs any animation involving the root bone, some of the torso bones or, of course, the arm holding the glass, in a way that its angle rotation relative to the Z-axis is altered.
As for the drinking bird, it's a fascinating contraption and much more than a simple toy. Apart from the principles it's based on, its apparent behaviour is mighty interesting on his own. It does work as a relaxation oscillator with a very long period, well over the dozen of seconds, with a transient dampened oscillation of a much higher frequency superimposed to it immediately after the relaxation event is triggered, followed by a long ramp. Graphically, it would go like this (alpha: angle formed by the longitunal tube in relation to the surface the birdie is resting on):
Right now, I'm not sure about the kind of function representing the value of alpha in function of the time, for both its ascending and ramp portions. The transient oscillation, I guess, it's a dampened sine function.
BTW; this type of oscillators are very commonly found in natural dynamic systems (heart pacemaker, dripping, to just mention a couple of them).
Of course the animation included only simulates this behaviour in a very crude way, but, hey, it's free! Also I didn't want to waste more than a few hours with this silly thing.