In fact, they found that birds that used numbness for only a short time could lose up to 15% of their body weight. Birds that took longer breaks, on the order of 12 hours, only lost 2%. Birds that reached lower temperatures also lost a smaller percentage.
Some species, such as glittering purple, have dropped to a set temperature (in this case, around 46 degrees Fahrenheit) regardless of ambient temperature. Others, like the black metal tail, seemed to follow the air and were very cold. A metaltail hovered around 38 degrees Fahrenheit, marking the lowest recorded temperature of any hummingbird, as far as researchers know.
These birds have been kept in captivity overnight, but Dr. McKechnie says he thinks there is more to be learned in a natural setting about how hummingbirds conserve energy.
There are stories of hummingbirds in the Andes that will enter a cave during cold spells and not emerge for several days, a pattern that, if confirmed, would suggest the birds are capable of hibernating, he notes. Similar to numbness, hibernation saves energy in an organism, but lasts longer than a single night.
“For me, the next step beyond this study would be to get a clear idea of where they perch,” said Dr. McKechnie.