Complex sentence: Ziggy's question as to why ducks don't leave their cages when they are free to do so, a metaphor for the docks institution in that even if given the chance to leave many Stevedore's would not because they know nothing else.
Split into simpler sentences: Ziggy's question as to why ducks don't leave their cages when they are free to do so. A metaphor for the docks institution; even given the chance to leave many stevedores would not because they know nothing else.