Thanks to Jack Saba for pointing me to the FWS Feather Atlas [
https://www.fws.gov/lab/featheratlas/ ] and a suggestion of whip-poor-will. I segued to Common Nighthawk, which was more likely and it seems a good match. What happened? We'll never know, but likely a successful Cooper's hawk attack if in twilight, or if after dark, a Great Horned owl would be implicated. Only these and a few other scattered feathers remained.