Just like humans Cockatiels can sneeze as well. Some sneezes are just as in humans normal, like maybe some dust in the air, but you should keep a watch if the sneezing is frequent on your cockatiel.
Cockatiels are little, so sneezing is not laud and might be difficult to hear, so be on the look out as frequent sneezing might be an early sign that your bird might be getting sick. Humans will cough when sick, in cockatiels you will probably hear a "click noise."
Another cause for your cockatiel to have a runny nose is lack of Vitamin A. This deficiency can be corrected by increasing the food quantities that are rich in vitamins and minerals.
Sometimes a cockatiel can have a runny nose if foreign particles like dust, food or seed particles are "stuck" on your cockatiels nostrils, this can also cause sneezing as well as a runny nose. For most parts a runny nose can also be a sign of infection such as a virus or bacteria. You can use a flashlight to check your tiels nostrils for any foreign particles.
Cleaning chemicals used to clean around the house can also cause sneezing and a runny nose. Make sure your cockatiel is not around any type of smoke, air fresheners, potpourri, as these types of products can all be irritating to your cockatiel's respiratory system.
If your bird is all fluffed up, acting lethargic, along with a runny nose or sneezing, you should seek veterinarian attention as soon as possible. It's very important to keep your bird warm while transporting her to the vet.
If your cockatiel occasionally sneezes a clear fluid (less than once or twice a day) and has no other symptoms, your cockatiel is probably just exercising the normal mechanism to clear his nostrils.
If discharge continues, or if it looks like anything other than a clear, thin fluid, or the sneezing is persistent, or other symptoms are present, then you need to consult your veterinarian.
As always the information offered here is to provide guidance and is not intended to be a substitute for the good advice provided by your own avian vet. When in doubt always consult your own veterinarian.