See
http://en.wikipedia.org/wiki/Screech_owl .
Screech owl is sometimes used to mean barn owl but usually denotes a separate kind of owl from the barn owl. Unfortunately this kind of owl is a kind of American owl.
I suspect that Tolkien borrowed this distinction from some account associated with the New World and himself did not know the distinction.
The barn owl (
Tyto alba) is found almost worldwide. The screech owl, (
Strigidae) belonging to the genus
Megascops, is limited to the Americas.
See
http://www.inkart.com/pages/nature/owls.html