I still am sticking to my idea that is a hawk, for even a full grown amaracaun(sorry my spelling is bad) is usualy donly three to four pounds max; being only six months old, is not fully grown, would weigh enough for a hawk to scoop it away
My guess would be like you figured hawk or eagle. I only say that because of there is no sign of any remains. do you have a lot of hawks around? another thing I wonder is if you have a lot of brush and "stuff" in your yard. Last year I had a similar incident and thought something got my bird...