What are the central themes of Birdy?

There are a lot of themes that run throughout Wharton's book.  
One central theme has to be about dreams versus reality.  Birdy creates his own birdlike existence as a way to escape his own abysmal reality.  To Birdy and Al, the real world is a place to be escaped from.  All of the places where the boys should be receiving love, guidance, and structure are places of harshness and cruelty.  In his dreamy bird world, Birdy is able to escape from nuns, teachers, parents, and army commanders.  The theme of escapism could also be a central theme that coincides with the dream versus reality theme.  
A second central theme has to be friendship.  Readers are introduced to Birdy through Al.  As Al's memories become blurred with Birdy's memories, it is made clear to readers that their friendship is just about the only part of reality that either of them can stand.  Their friendship is a deep bond that is strengthened by their time suffering through similar situations.  
Love seems to be another central theme.  Although Birdy's love for Petra is a fantasy, his feelings for her are no less real.  Through his imaginary relationship with Petra, Birdy is able to experience the love and security that he longs for in real life.  The relationship becomes so real to Birdy that he has a physical reaction while dreaming that he and Petra are mating.  His love for her is deep enough that he experiences real tragedy and mourning when she dies.  
Finally, I think a fourth central theme has to be the horrors of war.  This book doesn't make any attempt to glorify war or violence.  Wharton doesn't shy away from graphic descriptions of the war, and he shows readers that war leaves men deeply scarred both physically and emotionally.  Readers are told that Al wants to get sent back to the United States on a "psycho discharge" because the things that he has seen have made him "scared of even being scared."