@APL Their head rotation is a remarkable but not entirely unique feature of the owl family, as is nocturnal hunting, and the various evolutionary adaptations within the neck - so that they do not rupture blood vessels when rotating their head, for instance -are pretty remarkable. Other raptors can demonstrate very similar levels of mobility with their necks - the red -tailed hawk would be an example. And the reason why owls have adapted to have this neck flexiblity is because their eyes are largely fixed within their sockets.
Nice article on neck rotation in the national geographic;
http://newswatch.nationalgeographic.com/2013/02/06/how-owls-twist-their-heads-almost-360-degrees/
But that apart, their gross physiological features place them firmly within the bird family.