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.