My two recent obsessions have been hummingbird pee… and torpor

Hummingbirds are tiny (and I mean, tiny) birds.

They use up energy very quickly and barely store any fat, so they really don’t have a backup generator to rely on if they come close to running out of fuel. I’m really interested in how they manage this limited energy over short time scales.

Video: Hummingbirds may be small, but their energy consumption is huge. Learn how they do this.

Okay, so going back to my obsessions. We all know what pee is, but here’s a hummingbird peeing, just in case you were curious:

A Rivoli’s hummingbird peeing as it is released. Photo credit: Don Powers

We injected stable (non-radioactive) double isotopes of water (Deuterium + Oxygen-18) into hummingbirds, and collected their pee just after injection, then released them. Twenty-four hours later we would try capturing the same hummingbirds to collect another pee sample. The change in the levels of isotopes in their pee over 24 hours told us how much energy they’d used in the wild in that time! How cool is that? So I was very excited when they peed for us. Sometimes, as you saw in this gif, they would pee as a goodbye token to us as they were released. This was often even more exciting, because it could be the only pee sample we got from that bird– we would scramble to collect it from our hands!

On to the second obsession – what is torpor?

We use infrared cameras to measure hummingbirds’ surface temperatures at night. Here’s a hummingbird spending energy to keep its body temperature nice and high at about 41oC (though because of the insulation its feathers provide, its surface temperature maxes out at about 36oC):

A hummingbird maintaining a normal body temperature. You can see it breathe! Video credit: Anusha Shankar, Isabelle Cisneros, Don Powers

We also use oxygen and carbon dioxide analyzers to measure a hummingbird’s breath at night and estimate how much energy it spends per second. A hummingbird at normal body temperature spends energy something like this for an hour at night:

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Hummingbirds in Torpor

At night, hummingbirds can’t see or feed, so they use torpor – torpor is an energy-saving mode, a form of hibernation, that some animals use. Like humans, hummingbirds are endotherms; they generate their own body heat to keep warm. By using torpor at night, some endotherms allow the outside air to decide their body temperatures, and allow some of the internal processes in their body– their metabolism– to slow down. These torpid animals can drop down to using just 5-30 percent of the energy they would normally use while awake and resting.

Hummingbirds, being tiny, speedy, flying machines, often use torpor overnight, when they don’t have access to their energy-packed sugary nectar. Somewhat dramatically, they do this to avoid speeding their way to death overnight. But as a result, hummingbirds in torpor are quite useless; a torpid hummingbird cannot respond to outside stimuli for between 10-20 minutes. Here’s what it looks like when a hummingbird enters torpor (black means the bird is at normal body temperature, and red means the bird is in torpor):

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And when it is in torpor for a whole hour:

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And this infrared image below is what a hummingbird in torpor looks like- you can barely see it!

When you hit play, it will look like it isn’t playing, but you’ll see the color change slightly- it is playing! You’ll notice that its body’s temperature is about the same as that of the air around it (~ 17oC)! Video credit: Anusha Shankar, Isabelle Cisneros, Don Powers

Imagine if you were on a cold mountain somewhere, running out of food, and you could turn your internal thermostat down and save energy–all without feeling terribly cold, because your body was itself cold! I was mindblown when I realized that hummingbirds can save 65-92 percent of their energy every hour that they use torpor. I am so happy that scientists before me invented ways of measuring the oxygen in a bird’s breath, and ways to measure the temperature of a surface with a camera. And that I was able to take this technology to the field and explore what hummingbirds in their natural habitats do to balance their crazy energetic needs! This capacity some animals have to turn down their thermostat can help us understand animal and even human metabolism better. Maybe eventually we can figure out how to make humans torpid and send ourselves to space for decades!

Beautiful Places to Work

My field team and I have gone to a number of beautiful and beautifully different places to study how hummingbirds manage their energy on a daily basis. Here are some of our sites:

Patagonia Lake State Park, Arizona, USA. Photo credit: Anusha Shankar

A view from the Santa Lucia cloud forest in Ecuador (1900m). Photo credit: Anusha Shankar

Maquipucuna cloud forest, Ecuador (1400m). Photo credit: Anusha Shankar

A view from El Gullan, Universidad del Azuay, in the Ecuadorian Andes (3000m). Photo credit: Anusha Shankar

Our hummingbird headquarters at El Gullan, Universidad del Azuay, in the Ecuadorian Andes. Photo credit: Anusha Shankar

Mist nets we used at El Gullan, Ecuador, to catch hummingbirds. Photo credit: Anusha Shankar

In the video, a description of our site at El Gullan (owned by the Universidad del Azuay), near La Paz, Ecuador.

This story first appeared on the National Geographic blog.

Anusha Shankar took a while to decide what she wanted to be- and is realizing there will never be a final answer to that question. It used to be one of: doctor, teacher, pilot, actor. She is now a PhD student in Ecology and Evolution at Stony Brook University, and a National Geographic Explorer. She studies how hummingbirds balance their energy expenditure on a daily basis, in the deserts of Arizona and the Ecuadorian Andes. She loves salsa dancing, reading fiction, mentoring students, and walking around new places. For more on her research, visit: