Birds chirping at night? ?

This week these birds that must have a nest in a tree outside my window have been chirping late into the night. It only started this past week and I'm not sure what kind of bird would be up this late because the chirping doesn't start until around 11 pm. I live out in the country in the piedmont region of NC on a road that isn't too busy at night. There isn't any light except for one street light and the moon so I don't know why the birds would be out this late. Any ideas what type of bird it might be?