Doodle and the Scarlet Ibis are both in environments that are not hospitable to them. Doodle's environment includes the natural world, his brother, family, and social world. Doodle probably would have been okay in his environment if Brother had not pushed him so much. But Brother did this because he wanted so badly (for selfish reasons) for Doodle to be like everyone else. He wanted Doodle to be "normal" in order to fit in.
When they find the ibis in the tree, a feather drops to the ground. The ibis and Doodle are parallel characters. This means that they are similar in many ways. The feather dropping to the ground illustrates how vulnerable and fragile the ibis is. Likewise, Doodle is vulnerable and fragile.
The ibis is from more tropical climates. The father assumes that a storm had blown it off course and up to the north. Being out of its more natural, tropical climate, the ibis is like an outcast in its new northern environment. Likewise, Doodle is somewhat of an outcast in his own environment.
Brother can not stand that Doodle is so different from other children. In his desire to have a "normal" brother, he tries to force Doodle into fitting in. He trains Doodle and pushes him so that he might be more like other boys his age. For Doodle and the Ibis, being forced to assimilate and adapt to the environment proves to be too much. While the Ibis and Doodle are forced to adapt physically, Doodle has the added pressure of dealing with the social pressures of fitting in. However, it should be noted that these social pressures are mainly forced upon Doodle by Brother.