Predation.

The Pied Currawong is quite a striking bird that used to be mainly an autumn visitor, but nowadays the odd bird is likely to be seen at any time of the year. They are quite predatory, and last season the New Hollands lost at least one fledgling  to a prowling currawong. Recently there was a commotion in the carport,  a currawong had nailed a young blackbird not long out of the nest. It took it across the drive and proceeded to have a hearty meal.
Click pictures to enlarge.