If you have read Small Gods you'll get this without explanation If you havn't then it goes like this.

Eagles are the only creature that have figured out how to eat a tortoise. It picks up the poor lil fellow, flies high then drops it to break the shell. The casual observer may notice that there are easier things for an Eagle to eat, so no one really knows why Eagles do it, except the Eagle. Feel free to speculate in comments

The tortoise in question is the Great God Om, who one day thought he would turn himself into a magnificent white bull, but ended up as a tortoise. At which point he discovered he could not change himself back. Thus the story starts.

Hope ya like it

John