Instead of burying them and hoping that things in the earth would clean them up I tried maceration. The last lot of skulls I tried cleaning by burying ended up with bits missing and the beak sheaths disintegrating. It also takes a long time. Maceration is fairly quick. It involves leaving them in warm water for a week or so. I won't go into too much detail but it has been a bit more hands on and smelly. Worth the effort though as my skulls have just about survived intact. One thing I have learnt is don't keep them in red fishnet stockings. The idea was to keep them separate and not loose anything. Never occurred to me they would soak up the die. Even after a bit of bleach they are still very pink!
So here is my complete collection of bird skulls. Clockwise from top left: barn owl, buzzard, wood pigeon, kingfisher, house sparrow, linnet, song thrush, blackbird, shag bits x 3, shag x 2, oystercatcher, herring gull x 2, kittiwake, fulmar, razorbill, guillimot, jay x 2 carrion crow, rook and tawny owl.
And now I need a new box.