Tremor and rigidity are indications of a possible seisure .
Convulsions or seisures may start as spasms of the muscles,these would occur
in almost every part of the body.
Seisures can be seen in many forms,perhaps just a minor spasm in a leg,the
difficulty in trying to perch,the odd fall.
A full blown seisure is terrifying for the bird and the owner,both you and
your bird must be kept calm.
Seisures can indicate several problems and any information you can supply
the vet will be very important.
Possible causes could be hypothermia,poison,epilepsy,infectous disease or
metabolic disorder.
Any bird found in this condition must be placed in a warm dark and quiet area.
A vet must be consulted asap.