In Things Falls Apart, explain the metaphor of comparing Okonkwo to the nza bird who forgot himself after a heavy meal and challenged his chi?
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 well for themselves, they might begin to think that the rules don't apply to them anymore. Both Okonkwo and the nza bird were comfortable and stopped thinking about the possible punishment they might receive because they believed themselves to be invulnerable. It should also be pointed out that Okonkwo had a quick temper—and we can safely assume the nza bird did as well—to go on to pick a fight with their chi.
The third possible meaning to the metaphor is that they were selfish. Community obligations were very important to their faith, and Okonkwo especially brought misfortune to his whole tribe with his mistake. In that way, both the nza bird, and Okonkwo only thought of themselves. Their faith taught them to live at peace with their chi and the other gods, but it was more important to them to express their might and anger, knowing full well that they were in the wrong.
The incident where this comparison is made comes in Chapter Four of this great novel, after Okonkwo has broken one of the many tribal taboos by breaking the Week of Peace and beating his wife. Although by his own cultural standards, his wife was at fault, whatever she did the Week of Peace is a sacred time for his tribe, when the whole tribe lives in peace to honour the earth goddess, as is made very clear by the priest:
"You know as well as I do that our forefathers ordained that before we plant any crops in the earth we should observe a week in which a man does not say a harsh word to his neighbour. We live in peace with our fellows to honour our great goddess of the earth without whose blessing our crops will not grow. You have committed a great evil." He brought down his staff heavily on the floor. "Your wife was at fault, but even if you came into your obi and found her lover on top of her, you would still have committed a great evil to beat her."
Note the severity of what Okonkwo has done. His actions threaten the future food source of the entire tribe. This is why everybody talks about his action and we are told that his enemies compared him to the nza who stupidly challenged his personal god or chi. The comparison here seems to deliberately make Okonkwo out to be worse than he is. He is certainly no "little bird" in his tribe, and the idea that he is stupid enough to forget his religion after a heavy meal is deliberately demeaning. And yet there is some value in the metaphor, because Okonkwo did allow himself to be overcome by his anger, even when he knew he shouldn't.