The sacrifice of Mundo Cani, since Chauntecleer had cursed at and hated him moments before his last brave act. He even uses the dog'sCatch Phrase out of sheer regret and misery.
Chauntecleer's sheer brokenness before the battle with Wyrm. He's completely lost hope, and it shows.
Lord Russel's death in The Book of Sorrows, especially since it is so slow and Chauntecleer is so desperate to save him that he feeds him as long as he can, until he realizes that he's feeding a lifeless body.
The deaths of Ferric's family despite all of John Wesley Weasel's efforts to save them.
The corruption and death of Chauntecleer, and his final moments with Pertelote. He is unable to bear the idea that Ferric Coyote forgave him for the deaths of Rachel and his pups.
The backstory of the Bird With No Tongue, who was fooled by Wyrm into drinking his essence and lost her voice to it.
Chauntecleer, hoping to rescue Mundo Cani and bring him to the surface alive, discovers that he is much too late and that the dog is dead when he finds the skeleton deep in the earth.
When he is infested with parasitic worms that add to his worst qualities, Chauntecleer becomes violently paranoid and drives away all of the animals in a rage. They don't return until the end of his life.