The poem is indeed a complex one, and there will be more interpretations out there, I'm sure.

- A common-sense / thumbs-down on the Dodo's mode

Here I think Plath is recognising the innate instinct and zest for survival that a young child has. She would marvel at this as, particularly for someone who harbours suicidal tendencies herself, it is a blessing. The child as a human will not follow the 'Dodo's mode' and will be a survivor.

-Trawling your dark as owls do

This works as a reference to the fact that babies do not have regular sleep and may seem nocturnal. They are alert at unusual times, seeking attention. Of course this could also be the baby in the womb.

- father off than Australia

 represents the boundary of understanding which still exists between mother and child, although it is usually the closeness of this relationship which is celebrated. The child is another being, complex in its motivations and not clear in its communication. A frustrating distance can be perceived in a mother trying to understand her young child's demands and being unable to do so. This could also be seen as an exaggeration of the physical distance which a pregnant mother can feel from her child before it is delivered.