Migrating birds and energy-saving engines

By: Joanna Tien

Exercise. Working your muscles, getting your heart pumping, leaking salty liquid from your pores for the hope of burning off the clingy layers of lipids that keep sneaking in when you’re not looking. Lipids, or fats, make for great energy stores and make even better emergency energy generators, but they are only one of three types of fuel your body can make use of. Proteins, the molecules that make up bodily tissues like organs and muscles, are understandably vital for continued bodily function but also act as fuel for physical exercise.

Few feats are as energy intensive as long-distance migrations, and for birds it can involve flying non-stop for up to half a day or more. Our avian friends are well-known to burn proteins alongside fats to fuel these long trips, but how can it be a good idea to burn up muscle and organ tissue when you need them to fly? A team of scientists led by Alexander R. Gerson from the University of Massachusetts Amherst, USA, set out to determine how burning organ and muscle mass for fuel, also known as lean tissue catabolism, affected physical performance in migrating songbirds.

The team of scientists conducted their study with migrating yellow-rumped warblers, Setophaga coronate. Lowered humidity levels in the wind tunnel were discovered to increase the amount of lean tissue catabolized, regardless of the duration of exercise, compared to flights in wetter conditions. Measurements of the birds’ bodily condition and performance was taken before and after they were flown inside a wind tunnel for up to 10 hours. The speed at which bodies burn fuel for energy is known as the metabolic rate (MR). Peak metabolic rate (PMR) is measured when an animal is exercising and is frequently used as an indicator of physical performance ability, while resting MR (RMR) is measured when an animal is at rest. Both measurements were expected to decrease over time at similar rates as lean tissue catabolism occurred, since less energy is required to power a lighter body with fewer muscle mass, but smaller muscles mean less force generated.

What Gerson and his team found, instead, was that the bird’s RMR decreased much more than the PMR, hinting that this change wasn’t just because of weight loss. The researchers think that lean protein catabolism is not something used for energy-saving purposes, and that birds have a way to lower their RMR while at rest during migrations. Lowering RMR at rest could be a method of saving energy through decreasing the internal temperatures birds must maintain during the cold nights while on migrations.

Gerson and his team’s research can show us many important messages. Firstly, that simple changes in our surroundings can dramatically alter a situation, like when lowering the humidity in wind tunnels increased the amount of protein birds burned during exercise. Secondly, it shows us just how precious energy is; birds are only one of many animals in the world capable of switching to ‘low power’ modes during hard times. Lastly, it shows us that one problem can have many solutions, similar to how mammals react in the opposite way from birds by increasing MR after exercise.

To conclude, a team of researchers set out to figure out if burning proteins affect physical performance in birds and found that no, it did not. Instead, they discovered what could possibly be an energy-saving mechanism in migrating birds; that lowering RMR can save energy through cold nights. As is the case with many scientific studies, Gerson and his team’s research answered one question and brought up a whole host of new questions to answer. What else can we learn about the tiny, immaculately designed engines of the well-traveled feathery creatures we call birds?

Gerson, A. R., DeSimone, J. G., Black, E. C., Dick, M. F., Groom, D. J. (2020) Metabolic reduction after long-duration flight is not related to fat-free mass loss or flight duration in a migratory passerine. Journal of Experimental Biology. 223, jeb215384.