Ideas:

1. The head is in the dark, the sign of darkness also showing what looks like waves. I.e. the bird has its head in the watery abyss.

2. The watery condition is also alluded to with the undulating wings.

3. The moon returns to its dark phase to be reborn from the watery underworld later.

4. Two glyphs later we find another indication of which phase is meant:

We saw this type of glyph in what I believe is Friday. I have now taken the opportunity to make a comparison between these glyphs as they look in Keiti. At the end of Rakau the spirits move up into the mountains.