External parasites are more common in birds than internal parasites and are usually various species of mites. Regular grooming gives you the chance to check for signs of external parasites.
Any infestation of mites will require a trip to the veterinarian, so keep your bird's cage clean. For smaller cages, completely wash the cage and scrub perches and any corners with a mild bleach or detergent solution. Then rinse repeatedly. If possible, allow the cage to dry in full sunlight.
Common External Bird Parasites
Cere or Knemidocoptic Mites
Red or Gray Mites