Your cockatiel might show breathing difficulties for a variety of reasons. If the weather outside is too hot or your tiel is in a room that is too warm your bird will show signs of breathing difficulties or "short breathing." Leaving a cockatiel with unprotected sun exposure and no shading can be deadly. Also if your cockatiel gets scared or is very afraid you will also notice rapid breathing and sometimes with difficulties. All these behaviors are normal.
Other reasons for breathing difficulties are respiratory infections, toxic exposure, including Teflon poisoning and most kitchen fumes have also caused birds to experience respiratory distress. A cockatiel that is overweight and does not exercise very much can show signs of breathing difficulties or 'heavy breathing.'
"If your cockatiel is truly having trouble breathing, there is probably a serious obstruction in the chest, or lungs. Also an enlarged tumor in the chest area can cause breathing difficulties."
The most frequent cases of breathing difficulties in cockatiels are related to respiratory infections, you do need to see an Avian Vet as soon as possible for treatment. Respiratory infections sometimes are accompanied by a runny nose and sometimes sneezing.
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.