THE REASONS WHY:

Possums eat vast quantities of vegetation, directly competing with birds for available food.  They prey directly on adult birds on the nest, chicks and eggs.  They are scavengers and will prey upon anything.

Rats & mustelids (stoats, weasels, ferrets), prey directly on adult birds, chicks, lizards and insects.  Ship rats in particular are excellent tree climbers.  

 

Hedgehogs and feral cats prey upon anything they can catch.  Hedgehogs eat eggs from the nest.   Cats eat all birds, lizards and insects.

Mice and rats eat seeds, this slows the regeneration of the bush