There are several layers of meaning to the metaphor of the nza bird. One way to look at it is as a criticism of arrogance, superiority, and selfishness.
Concerning arrogance, it is clear that both Okonkwo and the nza bird foolishly challenged an adversary greater than them —their chi, a personal god. It's not an uncommon theme in fables and often has a lot to do with being well off. People who have come into good fortune start to believe that they are suddenly more worthy, possibly even more so than those who gave them that fortune. According to their faith, Okonkwo and the nza bird owed their good life to their chi. The nza bird had a "heavy meal," a sign of wealth, and Okonkwo was a respected man in his own right. The metaphor could mean that they both forgot to be humble and thankful, which leads us into the topic of superiority.
Following from the deduction that neither the nza bird nor Okonkwo were poor, you could say that the metaphor points our their superiority. When someone does...
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