The ibis is this story is not in its natural habitat. It is alone, and it stands out because of its shocking and uncommon coloring. It struggles because the enviornment of the boys hometown is not the weather it is used to - it finds the conditions harsh, hard to manage.
The same is true of Doodle. He was born disabled. He is unlike his brother and his family. He is alone, alienated, and the environment does not suit him - it is designed for those who can walk. Like the bird, Doodle is fragile; but also like the bird, Doodle is beautiful. His obstacles have made him appreciate life more than others. He is sensitive and pure. Although his brother teaches him to walk, Doodle can't overcome the harshness of his own environment, and is left behind in his brother's anger and rashness.
The author frames the story with images of the ibis to make clear the symbolism. The red color of the bird helps to further represent Doodle's mortality (blood) and the passionate emotions (rage, selfishness and love) that drive the protagonist's actions.