Ruby-throated hummingbirds redeploy energy-saving strategy they use to survive overnight without food to build energy stores for migration – ScienceDaily

Ruby-throated Hummingbirds use the same energy conservation strategy to survive nocturnal fasts and build the fat stores they need to fuel long migrations, shows a study published in eLife.

The findings help prove a long-standing distrust among scientists who study hummingbirds. They also provide new information on the rules birds use to determine whether to conserve energy or store fat.

The tiny ruby-throated hummingbirds constantly eat sweet nectar to fuel the rapid wing movements that keep them soaring. To conserve energy during their nightly fasts, birds can switch to an energy-saving mode called torpor by lowering their body temperature and slowing their metabolism by up to 95%.

“We wanted to know if hummingbirds use this same energy-saving mechanism to more quickly build the fat stores that they will use to fuel their 5,000-mile migrations between their North American breeding grounds and their homes. winter in Central America, ”says first author Erich Eberts. , doctoral student at Welch Lab, University of Toronto Scarborough, Ontario, Canada.

To study how and when hummingbirds deploy this energy-saving strategy, Eberts and the team measured the daily changes in body, fat, and lean masses of 16 ruby-throated hummingbirds during three time periods: the breeding season, late summer when birds are preparing to migrate, and during the typical bird migration period. They also measured the birds’ oxygen uptake using a technique called respirometry to determine when they went into torpor.

During the breeding season, hummingbirds maintained lean body masses and did not enter into torpor until their fat stores fell below 5% of their body mass. This “energy emergency strategy” was usually deployed on nights when they fell asleep with lower energy reserves.

But in late summer, when birds typically increase their body mass by 20% to sustain themselves during the long migration, they stop using the 5% threshold to enter torpor. Instead, they go into torpor more frequently and at higher levels of fat. This allows them to conserve energy and accumulate fat even as the nights gradually lengthen. “We have shown that hummingbirds abandon the energy emergency strategy at the end of summer and start using torpor to build up the fat stores they need for migration,” says Eberts.

The authors add that knowing more about this energy-saving strategy may be important for the conservation of ruby-throated hummingbirds and other migratory bird species that are under increasing stress from climate change and loss. habitat.

“Our findings that hummingbirds can use torpor to cope with different energy challenges throughout the annual cycle are important to understanding the differences in how these migrating animals and others that do not use torpor might respond. to future environmental changes in food availability and temperature, ”concludes Kenneth Welch Jr., associate professor and acting chair of the Department of Biological Sciences at the University of Toronto Scarborough, and co-author of the study at the University of Toronto Scarborough. alongside Christopher Guglielmo, professor at the University of Western Ontario, Canada.

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