James Hurst served in World War II, he grew up in North Carolina, and he had an older brother.
All of these facts would have influenced the story. First of all, when a person serves in a war he will see a lot of death. When asked about his story, Hurst said he could not really say why he wrote it, but that it was about the tenacity of the human spirit. This would have been something he witnessed fighting in the war. He would have witnessed both bravery and death, and this story was his way of conveying those emotions. We see both the triumph of overcoming an obstacle, and the pain of watching a loved one die.
I began to weep, and the tearblurred vision in red before me looked very familiar. "Doodle!" I screamed above the pounding storm and threw my body to the earth above his. For a long time, it seemed forever, I lay there crying, sheltering my fallen scarlet ibis.
The scarlet ibis is a bird found in South America and the Caribbean. However, it is not impossible that one would have found its way to North Carolina, where Hurst grew up. If it had, it definitely would have made an impression! What James Hurst would have seen plenty of were birds, because North Carolina has wetlands. The American ibis would be seen there. When the scarlet ibis is seen in the story, it is also unusual.
Sadly, we all looked at the bird. How many miles had it traveled to die like this, in our yard, beneath the bleeding tree?
Finally, Hurst had two siblings. He was actually the youngest of three children, so he might have related more to Doodle than the narrator. Maybe he had an overbearing brother. Either way, he definitely had experience with siblings, and this would have helped him to write about sibling relationships. You do not have to have had sibling to write about them, but it definitely helps.