Update: After some five (!) hours an adult male attacked, jabbing ferociously until feeding from the opposite port. (Hummingbirds don't share and are examplars of Kissinger's saying about academia: The fights are so vicious because the stakes are so small.) The youngster hung upside down for a few minutes, motionless.
I thought he was injured or dead, but when I went outside to take a closer look he'd righted himself. Now he's just sitting there again, taking the occasional sip. Maybe he'll spend the night...
Further update, 6:40pm: S/he's gone--hopefully someplace safe for the night.