I want to make a bird (about 4 feet tall) so it would have long, thin legs. Would you be able to make an armature for that out of tin foil?

Any long, thin limbs suspended apart from the body should be re-enforced stronger internal supports (wire, rebar, steel, mesh). Especially if an arm or a freely floating lock of hair is unsupported on one end. There is a significant chance of breakage the longer and thinner the limb. 

When re-enforcing a sculpture with an armature, make sure that the PTP encases a large portion of the actual weight bearing supports. There is a reason for this. Ultimately in a few decades, the foil core will turn to dust and this will leave the sculpture unsupported. The armature risks rattling around in a big hollow sculpture without offering any strength.  A good solution is to press groves into the finished foil and lightly secure (hot glue, pins) the armature in these trenches. During the scratch coat, these groves are filled up and the armature is encased in the PTP, properly held well below the final surface.