Hummingbirds’ metabolisms are so fast that they face an ever-looming threat­­: running out of energy and dropping dead. That’s why the birds evolved a physiological trick, an ability at the centre of a  study* by researchers.

It’s called torpor – a state similar to hibernation, in which a hummingbird's body temperature drops 20 degrees or more and their metabolic rate slows by up to 95 per cent. The researchers suggest hummingbirds use this ability strategically. 

They're basically shutting off at night and then just waking up in the morning like they arose from the dead.

The researchers observed that about halfway through summer, the birds were going into torpor almost every night. But not long after, ‘the birds are suddenly doing something very different.’

In fact, torpor in hummingbirds does appear to follow rules – seasonal ones. In early and mid-summer, when male hummingbirds stay light and agile to defend their turf and court mates, they use torpor to survive so-called “energy emergencies.” The researchers’ study determined this emergency state kicks in when their fat levels drop below five per cent of their body mass.

The data help simplify this almost nebulous question of when hummingbirds use torpor; it's when they hit this emergency level of fat during the night.

But the really interesting part was that the five-per-cent threshold essentially disappeared in late summer right before the migration period.

The study found that when it’s time to migrate, the birds will enter torpor at high body-fat ratios, even up to 40 per cent. By eating more during the day and using torpor at night, the birds can better prepare for the journey south. Researchers have previously observed hummingbirds fattening up before migration, but whether it was due to torpor was unconfirmed.

Hummingbirds can use torpor in different contexts to support their seasonal energy demands. It's this really versatile energy manipulation mechanism.

The study suggests that torpor is more than an emergency fail-safe – hummingbirds can use it to meet different energy needs.

Footnotes:

*  https://elifesciences.org/articles/70062