It is not likely that your bird has fleas. Birds scratch their heads for many different reasons. When birds are molting the new feathers coming in cause them to scratch. Sometimes they have an infection or allergy that causes the bird to scratch.
If you are feeding colored pellets or cheap pellets that are full of sugar and chemicals, this often causes birds to scratch. If you are feeding table food, again that would be full of chemicals and unhealthy additives, it could be an allergic reaction.
Make sure you are feeding your bird either Harrison's or Roudybush pellets and Find an Avian Vet for your bird to rule out health issues if your bird scratches all day long.