The coffin Doodle's parents had built for him is a symbol/image that suggests the possibility of Doodle's death. When Doodle learns to walk, his cart is placed in the barn beside his coffin. This suggests the possibility that Doodle has progressed, but Brother pushes him too far.
The color red (scarlet) is a significant image in the story because it connects blood, the ibis, and Doodle (his blood upon his death). When the scarlet ibis falls to the ground and dies, Daddy reads from the bird book that it is native to Florida and South America. Like Doodle, the ibis has trouble adapting to its environment. When the ibis dies, Aunt Nicey notes that dead birds are bad luck, "'specially red dead birds." The dead bird died because it was out of its element: like Doodle. The image of the ibis is a parallel for Doodle.
Note the repetition of the color red (red, scarlet, and vermilion) which connects the images of the ibis, the "bleeding" tree, and death (the ibis's and Doodle's). Brother finds Doodle dead:
He had been bleeding from the mouth, and his neck and the front of his shirt were stained a brilliant red.
He sat very awkwardly, with his head thrown far back, making his vermilion neck appear unusually long and slim. His little legs, bent sharply at the knees, had never before seemed so fragile, so thin. I began to weep, and the tear-blurred vision in red before me looked very familiar.
The vision of red looked familiar: similar to the scarlet ibis, the image of red.