Are they doing this because they are bored? Are they doing this because they like each other?

Is it true that this behavior comes from the time that the bird was a youngster when they were tapping against the beak of the parent bird in order to give him something to eat? Is the birds' exhibiting this behaviour a continuation, but as for now to confirm and strengthen mutual ties?

Here is an example:

http://www.youtube.com/watch?v=q4cv-2OVm-0