Energy-saving strategy helps hummingbirds fuel their long migrations
Ruby-throated hummingbirds redeploy an energy-saving strategy they use to survive overnight without food to build energy stores for migration

Date:
December 14, 2021
Source:
eLife
Summary:
Ruby-throated hummingbirds use the same energy-conserving strategy
to survive overnight fasts and build the fat stores they need to
fuel long migrations, shows a new study.



FULL STORY ========================================================================== Ruby-throated hummingbirds use the same energy-conserving strategy to
survive overnight fasts and build the fat stores they need to fuel long migrations, shows a study published in eLife.


==========================================================================
The findings help prove a long-held suspicion among scientists who study hummingbirds. They also provide new insights on the rules the birds use
to determine whether to conserve energy or stockpile fat.

Tiny ruby-throated hummingbirds constantly eat sugary nectar to fuel
the rapid wing movements that allow them to hover. To conserve energy
during their overnight fasts, the birds can shift into an energy-saving
mode called torpor by lowering their body temperature and slowing their metabolism up to 95%.

"We wanted to know if hummingbirds use this same energy-saving
mechanism to more quickly build the fat stores they'll use to power their 5,000-kilometre migrations between their North American breeding grounds
and Central American winter homes," says first author Erich Eberts,
a PhD student at the Welch Lab, University of Toronto Scarborough,
Ontario, Canada.

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

During the breeding season, the hummingbirds maintained lean body masses
and only entered torpor when their fat stores fell below 5% of their
body mass.

This 'energy-emergency strategy' was usually deployed on nights when
they went to sleep with lower energy reserves.

But in the late summer, when the birds typically increase their body
mass by 20% to sustain themselves over the long migration, they stop
using the 5% threshold for entering torpor. Instead, they enter torpor
more frequently and at higher levels of fat. This allows them to conserve energy and build up fat even as nights get progressively longer. "We've
shown that hummingbirds abandon the energy-emergency strategy in the
late summer and start using torpor to accumulate the fat stores they
need for migration," Eberts explains.

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

"Our findings that hummingbirds can use torpor to cope with different
energetic challenges throughout the annual cycle are important for understanding differences in how these and other migratory animals that
don't 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
alongside Christopher Guglielmo, Professor at the University of Western Ontario, Canada.

========================================================================== Story Source: Materials provided by eLife. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Erich R Eberts, Christopher G Guglielmo, Kenneth C Welch. Reversal
of the
adipostat control of torpor during migration in hummingbirds. eLife,
2021; 10 DOI: 10.7554/eLife.70062 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211214150204.htm

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