Doodle is born with physical limitations. Brother notes that when Doodle was born, "he seemed all head" and had a tiny, shriveled body. Doodle was so weak, everyone assumed he would die. Doodle shows strength in surviving, but he is still severely limited in what he can do physically. Brother, in his zeal to have a "normal" little brother, does teach Doodle to walk. But in the end, he pushes Doodle too hard. Had he worked with Doodle gradually and allowed Doodle to improve at his own pace, Doodle would have made progress and avoided punishing his body.
At first Doodle can barely walk. When Brother teaches him to do this, he sees hope that Doodle can become the kind of brother he wants.
. . . I wanted more than anything else someone to race to Horsehead Landing, someone to box with, and someone to perch with in the top fork of the great pine behind the barn, where across the fields and swamps you could see the sea. I wanted a brother.
Like the ibis, Doodle is fragile and vulnerable. Brother expects too much. Brother trains Doodle for his own selfish reasons. He is ashamed of having an crippled brother. If Brother's intentions would have been aimed at helping Doodle help himself, he would have paid more attention to Doodle's limits. Instead, Brother tries desperately to make Doodle more like himself. Brother can not stand that Doodle is "abnormal." But Doodle is simply different and Brother can not, or will not, understand this. Brother lacks the patience to allow Doodle to develop gradually. So, he pushes Doodle well beyond his limitations and in the end, he does this to a fatal degree.