Question: What is the difference between the calls of Chuck-will's-widow and Whip-poor-will?
Answer: Both calls sound very much like the name of the birds.
Chuck-will's-widow (left) calls actually have a little "chuck" at the beginning (sometimes you have to listen hard for it) and the calls are often repeated slowly for a considerable time.
Whip-poor-will calls are not as strong and they are quite rapid, about one per second.
Arthur Cleveland Bent writes in his Life Histories of North American Birds that one night in Charleston, SC, he counted 834 continuous Chuck-will's-widow calls at about one every 2.5 seconds. One can see why we often get questions about how to stop a Chuck from calling outside someone's window all night.