What does the swallows dead body most likely symbolize ?
(the book : the happy prince )

The dead of the swallow most likely symbolizes the kind heart the swallow had that he gave his life for a good cause.  

Explanation:

The story shows us that the swallow did for poor people much more than humanity, he knew that staying during the cold time would cost his life but it was impossible for him to leave knowing that there were so many in need, so when he dies his body is taken by angels to demonstrate that God loves the creatures that do good to others, and the good work of the swallow became bigger through his memory.