These are typically stress marks. They usually indicate that the bird is not getting the proper nutrition. If you are feeding your bird a high quality pellet diet, like Harrison's or Roudybush, for 80% of it's diet, then your bird may be ill.
If you are feeding no pellets, table food or cheap ZuPreem pellets then I suspect nutrition is the issue. ZuPreem has lots of sugar which can cause yeast infections and can lead to digestive problems. Also all of the artificial ingredients can cause problems too.