Duckie question..

[CityChicker]

It could be a number of different things or maybe even multiple different things. Even where we keep our birds in a relatively urban area, we have foxes every single night, raccoons, skunks, and owls, not to mention domestic predators (cats and dogs). I am not sure what predators you have in your part of the world. Whole birds, dead but not eaten, to me the first thing I would think of would be skunk (in our area, not necessarily yours). A bird with its head eaten though makes me think owl or something. Foxes too will kill birds and not eat them, but they generally kill more than a few at a time (sometimes whole flocks) and at least some of them are eaten. It could be the things other people have mentioned as well, but I am not familiar with what an attack from those animals looks like since we don't see them here.