Other models
Birds
The birds are built by four basic objects: one loft for the body and three extruded splines for wings and tail. Despite the simple model, the moving controls and the animation are not really basic and took a few days to get it to work.
The bird itself is built around a dummy object that links body (unanimated), wings dummies (rotating on one axis) and tail dummy (rotating on two axis). Wings also have a bend modifier for obvious reasons.
The flight is achieved animating the soaring with small movements and then adding a few wings flapping, using a hierarchy of dummies to move it through the scene. One dummy moves straight, one other follows the up/down (related to the wings flapping) of the bird and the third add a random (keyframed) movement to simulate turbulences.
Since they are always seen from a distance, the simple geometry is not noticeable and somehow hidden by the continuous movement of each control dummy.
Rats
If birds are basic entities, rats are almost a joke. Imagine a sausage with two ears and a tail. No eyes, nose, legs. Add a lot of motion blur, a fast movement and magically you think you can see the missing parts.
The only animation controls for these rats are some rotating bones bending the body and some more bones used to give the tail a simulated inertial movements. Then everything is linked to a dummy that moves forward pretty quickly.