Anusha Shankar: Modelling energy budgets of hummingbirds

Dr. Anusha Shankar is a National Geographic Explorer and Young Leader, a Lewis and Clark Field Scholar, and a Postdoctoral Fellow at the University of Alaska at Fairbanks. In this insight, Dr. Shankar discusses her paper “Hummingbirds budget energy flexibly in response to changing resources”, how this work may be used for other species, and gives advice to fellow scientists.

About the paper

What’s your paper about?

Anusha Shankar + hummingbird. Credit: Julisa Ricart
Anusha Shankar + hummingbird. Credit: Julisa Ricart

Picture yourself as a hummingbird. You weigh 3 grams – that’s two paperclips. You have to stay light to be able to fly, so you barely store any fat. You live on the energetic edge, always finding food and storing just enough to stay alive until your next meal. What secret energy management strategies do you, as this tiny, high-energy, bird, use to exist in such diverse habitats as the deserts of Arizona and the high elevation Andes? How flexible is your daily ‘energy budget’ – the energy you spend on different activities, such as perching, hovering, and flying, over the course of a day? We used a variety of techniques to measure how much energy hummingbirds spend in a day, as well as the different components of this daily energy budget, and modelled how flexible hummingbirds are in budgeting their daily energy. We fed hummingbirds a (non-radioactive) double isotope of water, collected their urine, released them to their natural habitat, and tried recapturing them 24 hours later for a second urine sample. The difference of isotope levels in their urine told us how much energy they’d spent in 24 hours. We measured the oxygen and carbon dioxide in their breath (with a respirometer) to estimate the energy they spent on the different activities.

What are the key messages of your article?

Hummingbirds. Credit Sandhya Prabhat
Hummingbirds. Credit Sandhya Prabhat

We discovered that hummingbirds are much more variable in how much energy they use in a day, and more flexible in how they allocate that energy to different activities than we previously thought! At our sites in Arizona, they adapt their daily energy expenditure and daily energy budgets to the number and distribution of flowers in the landscape – their main energy source. Our model also helped us understand that hummingbirds can spend between 30-85% of their day hovering and flying. Previous behavioural observations of hummingbirds unanimously found that they spend most of their day (about 70%) perching, and only 30% of their day on hovering and flying, so our findings are exciting because they show that hummingbirds are capable of much higher levels of activity than we thought they were capable of.

About the research

What is the broader impact of your paper

Hummingbird in a tree.

We have developed an energy budget model that takes field measurements and/or lab measurements, and uses field information (food availability, temperature), to estimate how a free-living animal spends its day, in terms of time and energy. We have incorporated individual variability to understand how a whole population uses its environment. Individual variation is what natural selection acts on, so we need to start building models using individual values, rather than just species means. As physiologists, we have been collecting lab measurements of energy expenditure on animals for decades, but placing these lab measurements in the context of what an animal does in its native habitat is becoming increasingly important given how quickly our environment is changing. We hope that others will adapt our field energetics model with other species.

The key is to collect a comprehensive measure, like total daily energy expenditure in the field, and individually measure as many of the components that contribute to that total as possible (e.g. energy expenditure for resting, activity, sleep, etc.). Then, by adding up the components and modelling what you cannot measure, you match the two up. For hummingbirds, daily energy expenditure = (time*energy spent hovering + flying + perching) + (time*energy spent at night) + (time*basal (maintenance) metabolic costs). For our study, we could not measure the time hummingbirds spent hovering, flying and perching in the field, but we knew all the other components in the equation. We therefore measured everything else, and modelled the time they spent on different activities. The model is more powerful if individual variability is incorporated rather than condensed (i.e. use the full range of values rather than only means).

What are the big questions still to answer?

We do not know whether there are broad ecological/physiological rules on how flexible daily energy expenditure is within a species, across taxa. How much can you as a human keep increasing your energy consumption and energy expenditure in a day? Is that capacity for a human proportionally the same as what a horse or a hummingbird can eat and spend? And what components of the energy budget are most responsible for that flexibility? How does basal metabolism relate to this flexibility? These are surprisingly unknown and open questions – some data and studies exist for specific taxa, but we do not have a broad understanding of animals’ energy management, especially in the wild.

About The Author

What do you do in your spare time?

Anusha Shankar (right) dancing. Credit: Carlos Murbach
Anusha Shankar (right) dancing. Credit: Carlos Murbach

I am obsessed with salsa dancing (and bachata, swing, and cha-cha)! I also enjoy mentoring, reading fiction and have recently started painting.

One piece of advice for someone in your field…

Collect data on yourself! If you are worried about your happiness, collect data on what correlates with your happiness. If you are worried about time management, collect data on what you spend time on (build an activity budget!). If it is your food consumption or weight that is bothering you, collect data on your energy budget (what your consumption and expenditure are)! I have been using phone apps to collect mood, time and activity data on my free-living self for about two years now, and having data can really inform your decision-making process. We all collect data for science, and we know it helps with policy-making; why not for our personal decision-making as well!

You can read the paper in full here or the free plain language summary here.

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