I feel pretty sure it is an owl. The "Screaming Chitters" call of the Great Horned Owl seems to sound as he described.
Rarely do song birds sing at night, except for the mocking bird.
http://www.owlpages.com/sounds/Bubo-virginianus-14.mp3
If you go to the site and play the above mentioned call it sounds like what Squirrelly is describing.