Indeed, they found that birds that used torpor only briefly could lose as much as 15 percent of their body weight. Birds who took a longer break, on the order of 12 hours, lost only 2 percent. Birds that reached lower temperatures lost a smaller percentage, too.
Some species, like the sparkling violetear, descended to a set temperature (in this case, roughly 46 degrees Fahrenheit) regardless of the ambient temperature. Others, like the black metaltail, seemed to be tracking the air and got very cold. One metaltail hovered around 38 degrees Fahrenheit, scoring the lowest recorded temperature of any hummingbird, to the researchers’ knowledge.
In fact, the metaltail, the black-breasted hillstar and the bronze-tailed comet, which are related species, all entered colder, longer bouts of torpor than the others. This could help explain why this group is more common at high altitudes — they have worked out ways to minimize the stress of living in an extreme environment.
These birds were held in captivity overnight, but Dr. McKechnie says he thinks that in a natural setting, there is more to learn about how hummingbirds save 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 that the birds are capable of hibernation, he notes. Similar to torpor, hibernation saves an organism energy, but it goes on longer than a single night.
“For me, the next step beyond this study would be to get a clear idea of where they roost,” Dr. McKechnie said.