Always a problem, CJB. Possums and rats, both unintentionally introduced, predate on eggs and nestlings, and stoats, weasels and ferrets, all deliberately introduced, presumably by madmen, predate on everything.
My gardening neighbours have wiped out the thrushes (introduced) with their snail pellets, you never hear birdsong in farming areas, particularly vineyard. Here in suburban Auckland, the only natives I see are silvereyes, a wren-sized omnivore/nectar-eater, and pukekos, a blue-black bantam sized omnivore with enormous feet. Tuis (nectar-eaters) and fantails (insectivores) are moving into residential areas, too, as bush disappears they are becoming urbanised, and everyone likes them. Australasian magpies take over parks, aggressive sods, and plovers like open grassland like parks. Seabirds in plenty, not very different from the European ones.
We get plenty of mallard, though. If you feed them, they will come indoors begging! Just a little way. All-but-one domestic ducks are descended from mallard, and it isn't surprising. They just beg to be domesticated.