Doodle was pushing himself too hard trying to catch up to his brother in the rain. So when he sat down next to a tree because he couldn't run anymore. He died leaning against that tree, all flimsy and small like that red bird that fell out of that tree before, which was foreshadowing his death.
Answer: In the end, Doodle, unable to live up to his brother's expectations, dies from exhaustion after collapsing while running after Brother in a storm. Doodle dies at the end of "The Scarlet Ibis" as a result of his brother's pride.
