One of the main issues of a cockatiel being overweight is that sometimes a cockatiel will be fed people's food that are too fatty, like potato chips, sugary treats, cheese or excessive amounts of peanut butter. It's important that your bird has a well balanced diet. Also, not enough exercise can make your cockatiel not burn enough calories.
The places to check to see if your cockatiel is getting too fat are the thighs, the chest and the abdomen. If your tiel is only getting fat in one area like for example around the chest area, that might not be necessarily that your tiel is getting fat, but it could be an illness like for example a tumor.
"Feeding your cockatiel greasy food like potato chips is always a bad idea."
There are many illness associated with obesity in cockatiels like having problems breathing or avian diabetes.
If your cockatiel is too fat, we strongly recommend that you see the advice of an avian vet before putting your bird on any diet, this is to make sure that your cockatiel is not suffering from any other illness.
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.