I've caught morning whoops similar to the two presented below in Georgia, and heard similar from other researchers in Michigan and Alabama. I've often wondered if they aren't an understated attempt by the vocalizers to keep track of each other shortly before they go to ground for the day light hours. Pure speculation on my part, but one day I hope we'll have the answer.
In the mean time, here are two morning whoops captured at 5:55 a.m. in Albemarle County, Virginia, on October 8, 2012. This video also demonstrates the ability of spectrograms to pinpoint quiet vocals, and for post processing to enhance vocals for better audibility.