Ideas:

This glyph - in the 5th period of the Moon - is different from the corresponding 'newly hatched' birds in the other periods. The beak is indicating the mature stage.

Perhaps the Full Moon just passed is the time when maturity is reached? Strange. No, I rather believe this fully grown bird belongs to the time of full moon.

But then this type of glyph (in all the periods) maybe shows how the bird of the preceeding period carries the moon over to the next period, like in a relay race. Though the receiving person is sitting waiting instead of starting to run.