Doddle is the only one who is empathetic enough to feel for the bird because it is weak, as he has been for his whole life.
When the family first sees the scarlet ibis, the beautiful bird is just sitting in a tree and falls down. Brother wonders how many miles the bird has traveled just to die.
Everyone in the family just sadly looks at the bird. However, Doodle is the only one who does anything about it.
Doodle knelt beside the ibis. "I'm going to bury him."
Doodle has had a hard life. He knows what it is like to be fragile. Because he is so fragile, he identifies with the bird. He feels for it and knows that he has come close to dying.
Doodle’s burying of the bird foreshadows his own hidden frailty and his death. Just as the bird seemed weak but not dead at first, Doodle appears to be not as weak as he is.