Published research shows that hypothermia is not at all the same as torpor in hummingbirds. Sleeping is an entirely different condition than either. If the hummingbirds sit on feeder perches to ingest cold sugar water and then fall or try to fly and in either case end up on the ground, they are in grave danger from predators.
In addition, female birds can't return to their nests to keep eggs and hatchling warm, so the entire family is lost if the mother bird becomes hypothermic from feeding while motionless on a feeder perch.
Even if they sit unresponsive on or hang from feeder perches, they are in danger of dying because the condition is hypothermia and the birds that are completely unresponsive because of hypothermia are said by researchers who are experts in hummingbird thermodynamics to be close to death. If the temperatures are cold in the morning or any time during the day, only feeders without perches should be used to keep from killing the birds you are trying to help.
Thank you for taking off the perches or only using perchless feeders if temperatures are below 60 degrees where you live.
The reference for the study that showed that torpor and hypothermia are two completely different conditions is
Torpor and hypothermia: reversed hysteresis of metabolic rate and body temperature
Fritz Geiser, Shannon E. Currie, Kelly A. O’Shea, and Sara M. Hiebert.
Am J Physiol Regul Integr Comp Physiol 307: R1324–R1329, 2014. First published September 24, 2014; doi:10.1152/ajpregu.00214.2014.
Just being asleep for a hummingbird is not the same condition as torpor or as hypothermia, so sleeping is a third condition.
This study showed that the hummingbirds in the study under controlled conditions with everything measured had their wing muscles adversely affected so they could not fly as well after drinking cold sugar water while remaining motionless on a perch.
FLIGHT THERMOGENESIS AND ENERGY CONSERVATION IN HOVERING HUMMINGBIRDS
PENG CHAI1, ANDREW C. CHANG1 AND ROBERT DUDLEY The Journal of Experimental Biology 201, 963–968 (1998)
Printed in Great Britain © The Company of Biologists Limited 1998 JEB1196
With some species of hummingbird in serious decline, everything possible should be done to keep them safe from harm.