What is the bone on top of a cassowary's head called?

The bony plate on a cassowary's head is called a helmet.

It is not actually known what the helmet is used for, but there are several theories. The most likely theory is that, because the helmet continues to grow throughout the cassowary's life, it could be a sign of dominance. Recent scientific research has suggested the helmet may even assist cassowaries in detecting the low vibrating sound made by other cassowaries.