Aww thats sad!
as mizkity says you done well taking care of him.
Its just the way nature is ! its mean sometimes
I rember i rescued a little sparrow from these magpies that where attacking him he was a baby i ran in between the magpies and grabbed him they where so visious!! i took him and put him in a box and my nana checked him over i called the wildlife place and they said he sounds old so i let him in my room to see if he could fly and he could they told me to keep him over night and let him go in the morning i did he flew of.
Next day i was out in the street and there he was lying dead under a tree the magpies had killed him
i felt so bad !!!