Probably more than one bird. If you go through a flock and suck in several birds under take off(full power) conditions its possible to knock a blade off. And once one blade goes its over..
http://www.youtube.com/watch?v=y2OS2pwrZTI
Heres what happens when a blade lets go..
http://www.youtube.com/watch?v=Ek6ad...eature=related