I am posting this for new owers, plus something they do:

Beak Grinding;

This is a sign of peacefulness and being content. Cockatiels often display this when they are getting ready for a sleep during the day or at night, even when just relaxing on your shoulder. It is a very gentle rubbing of the top and bottom beaků a bit like us grinding our teeth together, but in a good way. Often the little beard feathers are fluffed up as well.