Cats very often show symptoms of sickness in their behavior, and any sudden behavior change could mean that your cat is ill. (plainfieldanimalhospital.com)
Fortunately, birds are normally healthy and seldom show sudden behavior changes unless ill. (hartz.com)
Medical Emergencies — Rush your rabbit to the vet if you see: lack of appetite; diarrhea; few or no fecal pellets; listlessness; crusty ears; overgrown teeth, mucus around the eyes or nose; urine - soaked fur, straining to urinate; lump or swellings; head tilt; or any sudden behavior change. (laanimalservices.com)