Researcher examines hummingbird torpor in uncertain environment

Hummingbirds are extremely small but remarkably tough. Some species make migrations of over 9,000 km (about 5,590 miles) a year. Their journeys are largely fueled by nectar, which is sipped from wildflowers and converted to fat.

However, climate change and land-use change are making the distribution of wildflowers along their migratory routes more unpredictable. To buffer against these challenges, hummingbirds use a process called torpor, a sort of overnight hibernation. In new research published this week, Shayne R. Halter, a Ph.D. candidate at The University of New Mexico Department of Biology, investigates the rules for the maintenance of fat during migration in hummingbirds headed south in the fall through New Mexico. Hummingbirds apparently adjust their use of torpor to wake up in the morning with a minimum fat reserve that allows for survival on a day-to-day basis in an uncertain environment.

The research, titled The hummingbird's adipostat: can a simple rule explain torpor frequency and duration in hummingbirds?, has been published in Proceedings of The Royal Society B, a biological research journal. The research was conducted by Halter, along with UNM Professor of Biology Blair Wolf. Professor Emeritus Carlos Martinez del Rio of the University of Wyoming zoology and physiology department is a co-author of the research paper.

Distribution of wildflowers along migratory routes is often unpredictable, especially in the arid southwestern U.S. Ongoing climate and land-use change are contributing to this unpredictability. Due to these changes, migrating hummingbirds may become increasingly challenged to maintain adequate energy levels, Halter explained.

Torpor is characterized by a sharp decrease in metabolic rate and body temperature for several hours. This low-energy state is like hibernation but occurs over much shorter periods of hours rather than days or months. In his research, Halter looked at two species of migrating hummingbirds, Calliope (Selasphorus calliope) and Rufous hummingbirds (Selasphorus rufus), at a site along the Mimbres River in New Mexico.

"During torpor, hummingbirds are unaware of their surroundings, making them more vulnerable to predation and other risks. For this reason, torpor should only be used when needed. In my research, I use a combination of body fat and overnight metabolic measurements to determine whether hummingbirds are low on energy and how they compensate with torpor. The data I collected allows me to predict if and for how long these birds will enter torpor and how much energy reserve they require in the morning to sustain them until they can find their next meal," Halter explained.

He found that hummingbirds will enter torpor if their evening body fat falls below a specific threshold, where maintaining a high body temperature would cause their minimum morning fat reserves to fall below a critical value. In other words, Halter said, hummingbirds require a certain amount of fat in the morning to get them started toward their next meal.

"In our research on Rufous and Calliope hummingbirds, we found that torpor occurred when fat levels fell below 500 milligrams, and we found minimum morning fat reserves to be approximately 180 milligrams. Birds will only use torpor if needed and will adjust its duration to meet the minimum morning fat reserve," he said.

Explaining the significance of the research, Halter explained that annual migrations are demanding events for hummingbirds and are often accompanied by high mortality rates. The arid southwest is becoming warmer and drier, and these conditions are already affecting the timing and distribution of nectar sources. Additionally, land use change is replacing native plants with invasive and domestic, non-nectar producing plants. By providing a model that predicts torpor frequencies, durations, and fat reserves in migrating hummingbirds, this research will aid efforts to determine the ecological health of hummingbirds, define their energetic thresholds and limits, and formulate conservation measures.

Halter is a non-traditional student. He attended U.S. Air Force ROTC at the University of Colorado and received a B.S. in Aviation Management at Metropolitan State University of Denver in 1991. After that, he spent over 20 years in the Air Force as a navigator and flight examiner, completing 10 deployments to Europe, Southwest Asia, Africa, and South America, and received 27 medals and decorations.

"Several years before I retired in October 2013, I had decided I would use my GI Bill to go back to school and study science. Because my spouse, Susan Bogus Halter, is a professor in the Gerald May Department of Civil, Construction and Environmental Engineering at UNM, it made sense to attend school here. I was first exposed to biology while earning a B.S. in Anthropology at UNM, which I received in 2018," he said.

As part of this degree, he completed an Undergraduate Honors thesis on stable isotopes in chimpanzee teeth and their food sources with Associate Professor of Anthropology Sherry Nelson.

Wolf accepted Halter as a graduate student in 2019, and he spent the next two years taking classes and studying biology fundamentals.

"In 2021, Dr. Wolf suggested I do a hummingbird project and provided resources and a study site. At this point, I became completely committed to studying biology and hummingbird physiology. After two years of building equipment, conducting fieldwork, and analyzing data, I completed comprehensive exams and received an M.S. in Biology from UNM in 2024. One of the richest experiences I've ever had was building, calibrating, and testing the equipment I used in the field, then watching it collect meaningful data," he said.

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