From 'Six at the Beginning: the well is muddy' to 'no birds.'
The Sub-commentary states: Six at the Beginning 'dwells at the very bottom of the well, with no resonance above, submerged in silt and filth,' which means the bottom of the well is fouled with mud and unfit to drink; hence: 'the well is muddy, no one drinks.'
A well that is muddy and unfit to drink is 'an old well that has long gone uncleansed, which birds do not approach, let alone humans'; hence: 'an old well has no birds.'
On the note 'the well is an unchanging entity': The Sub-commentary states: As for 'the well is an unchanging entity, the seat of virtue,' the line judgment says 'the town may be changed, but the well cannot be changed,' hence 'the well is an unchanging entity'; the Great Treatise also says 'The Well is the seat of virtue,' hence 'the seat of virtue.'
The commentary states this to clarify that since the well has changelessness, it possesses constancy; since it sits in the seat of virtue, it exercises virtue.
Now dwelling at the lowest extremity, its constant virtue is utterly debased, so nothing takes from it, and birds and humans alike cast it aside.