'Metabolic knife edge': Study offers insight into hummingbirds' hibernation-like rest
Short-term hibernation, known as torpor, allows tiny birds to conserve energy between meals
Powered by nectar and a metabolic marvel, migratory hummingbirds travel thousands of kilometres each year, breeding in places like British Columbia before making the journey as far away as Mexico to avoid the winter cold.
To make it there, hummingbirds sometimes enter a short-term hibernation of sorts, slowing their respiratory, heart and metabolic rates for several hours at a time to a point where some birds have been mistaken for dead.
But rather than killing them, the process known as torpor allows the tiny hummingbirds to conserve enough energy to survive between meals.
New research is providing insight into when torpor happens in some varieties of hummingbirds common to B.C.
Lead researcher Shayne Halter, a PhD candidate at the University of New Mexico's department of biology, said the findings are an early step in better understanding how hummingbirds use their energy during migration and how that might be impacted by changes to the flowers they depend on for food.
"These hummingbirds are on what we call a metabolic knife edge, where they have to have energy, a constant flow of energy, or they just won't survive," he said.
"And in some areas, like where I live in New Mexico, their nectar supplies can be kind of unpredictable. And with land use change and climate change, we may be increasing that unpredictability."
The research, published in January in Proceedings of The Royal Society B: Biological Sciences, found that the Calliope and rufous hummingbirds require at least 180 milligrams of fat in the morning to get them started toward their next meal, so their bodies automatically enter hibernation mode once they hit 500 milligrams to avoid losing too much fat overnight.
Halter gathered his data from July to September 2022 at a site near Mimbres, New Mexico, but said it would be "amazing" to find Canadian researchers interested in building on what he has done.
"I suspect that, well, it's cooler up there, so those hummingbirds up there probably have higher fat thresholds. They'll probably go into torpor with more fat on their bodies than these hummingbirds down here, where I live," he said.
He considers his research an early step in understanding the limits of hummingbirds and mapping how they use their energy during migration, and said it would be interesting to do similar work over time to see how that changes.
"More research needs to be done," he said.
More than half of population lost
The 2022 State of the Birds report by the U.S. Committee of the North American Bird Conservation Initiative found that more than half the population of the rufous hummingbird has been lost since 1970.
If the hummingbirds can't find food, being forced into torpor can be risky, Halter said.
In the energy-conserving state, not only are the birds unaware of their surroundings, it can also impact their immune system, putting them at higher risk of illness or being eaten by predators.
"For the non-scientific person, [understanding torpor] is really for the sake of keeping these species alive and not allowing them to be threatened any further," Halter said.
New information welcome: rescuers
Jackie McQuillan, program director at the Wildlife Rescue Association of B.C., said any new information on hummingbirds is helpful in educating the public about how they can support the birds.
"So, in this particular scenario, knowing information about the fact that hummingbirds require a certain level of fat stores to remain active and not go into torpor helps us to give information around encouraging the public to provide good quality sources of food, things like local native flowers," she said.
McQuillan said the association takes in about 150 hummingbirds a year, including those injured while in torpor.
She said the majority of those hummingbirds are of the Anna variety, which do not migrate and enter torpor while living in the cold, but also include a smaller number of rufous hummingbirds.
For the average person without a biology degree, seeing torpor can lead to some surprising situations.
"We have definitely had people come in where they thought that the hummingbirds had passed away, and even during the car ride, then they start to hear noise in the box. Because as the bird warms up and starts to wake up they'll start moving again, and people are quite shocked by that," McQuillan said.
Alison Moran, director of the Hummingbird Project at Victoria's Rocky Point Bird Observatory, said Halter's research adds to the body of work explaining what her organization sees when it monitors birds on migration.
Moran said hummingbirds on their way to their wintering grounds get "massive," relatively speaking, growing from a few grams to 4.5 grams in some cases.
"It's really nice to understand how they're using torpor and fat deposition, and using it as an energetic calculator, really," she said.
"So, there's been good work in this area, but this really adds to it quite well."