Spotlight: A Shorebird Mystery

Scientific research often begins with a mystery – something in the data that makes the researchers think – Huh? That’s funny.

In this issue of Wader Study, authors Lindström and Alerstam begin their paper¹ with just such a mystery. The researchers studied the 5,000–7,000 km non-stop flights that Great Snipes Gallinago media make during migration from central Sweden to equatorial Africa (and back). To do this, they attached tiny sensors (called mini-multisensory accelerometers) to the bird’s lower leg (tibia). These sensors that weigh less than two grams and measure ambient temperature and pressure every hour and movement every five minutes².

The ability to collect data at this level of detail from high flying birds (literally migrating at cruising altitudes above 4000 m) is astounding. However, the researchers found themselves struggling to interpret some of the data. In several of the 15 flights studied, there were periods when activity suddenly stopped, and temperature rose. It looked like the bird had landed, but air pressure remained low suggesting the bird was still at high altitude. Then, usually in under an hour, activity was high again and temperature low. It was a mystery. After ensuring that the data were not due to a technical malfunction, the researchers started to ponder other explanations.

As previously mentioned, the sensor sits on the bird’s lower leg. Unlike birds that perch (Passerines), which tend to fly with legs tucked up against the body, most shorebirds fly with their legs stretched out under the tail. Maybe the unusual sensor data showed that the birds had folded their legs and tucked them under the belly feathers? This would explain the abrupt rise in temperature and even the decrease in activity, due to the angle at which the accelerometers measure movement.

This was a plausible hypothesis and it generated more questions: How common is it for shorebirds fold to their legs when flying? Which species do it? Are Great Snipes one of the species that do it? Then, delving even deeper into the mystery, why would the birds fly with folded legs? A literature search revealed very few studies on the topic. The researchers had their work cut out for them.

At this point, you might be asking why does this matter? Knowledge about the way shorebirds hold their legs when flying is not something to be immediately marketed. The scientists were simply curious. It is an example of fundamental research. There is no immediate application, but who knows where it could lead? Time and again it leads to later innovations, and it often starts with a mystery.

To try to solve their mystery, the researchers first put out a call for information over email and social media. They asked for photos and observations of shorebirds flying with their legs folded and any available information about species, location, date, and time. They examined online photos of flying shorebirds from the Swedish Species Observation System (www.artportalen.se). After checking nearly 10,000 photos, they found just over 1,300 that were sharp enough to determine leg position.

The data in this study were gathered mainly online. Working primarily online is something many of us have been doing since the start of the current pandemic (in the best-case scenario). Scientists have lost field seasons, experiments, even jobs. Though conducted before the pandemic, this study shows that research can go on, in some capacity, using data that are already available online.

 The researchers’ call for information produced reports of 17 species flying with folded legs.  The behavior was most common in the long-legged Spotted and Common Redshank species Tringa erythropus and Tringa totanus. Reports came from Canada, the USA, New Zealand and five countries in Europe, suggesting that flying with legs folded is not restricted to a particular latitude or longitude. However, the authors acknowledge that it is not the complete story because they lacked feedback from large parts of the world and therefore data on many shorebird species.

The 1,300 plus database photos with detectable leg position revealed at least 31 birds flying with folded legs. This more quantitative data allowed the researchers to estimate how often this behaviour occurs within a species. In Spotted Redshanks, about 11% of the photos had birds flying with legs tucked. Rates of over 1% were detected in Greenshanks Tringa nebularia, Ruffs Calidris pugnax and Common Snipe Gallinago gallinago.

All of the information together showed that shorebirds fly with legs folded in most months of the year, in different age classes, alone and in flocks, and in both migratory flights and local movements. So, why do they do it? Perhaps to decrease heat loss in cold temperatures. A similar strategy is used when birds rest standing on one foot and tuck the other into the belly feathers (unipedal roosting). Earlier research has shown that the likelihood of unipedal roosting increases with decreasing temperature³. The colder it is, the more birds stand on one leg when resting. Thermoregulation is clearly one reason to fly with legs folded, but this can’t be the only reason because birds also fly with legs folded when the ambient temperature is mild.

Might leg position affect flight performance? Folding the legs below the body moves the centre of gravity forwards. Whether this is a help or a hinderance might depend on how much fuel the bird is carrying. During migration, shorebirds can carry anything from no fuel stores to more than doubling their body mass with stored fat and protein. This fuel is mainly stored behind the centre of gravity, thus folding the legs could potentially balance stored fuel. Since birds carry different fuel loads, this might explain why both leg positions are found in flocks where birds are experiencing the same ambient temperatures.

Could muscle fatigue be another explanation? Keeping the feet trailing during long flights is tiring. To cope, shorebirds have fatigue-resistant slow tonic and twitch fibres in their leg muscles, whereas perching bird species, which tend to fly with legs tucked, have a lower proportion of such slow fibres⁴. Based on muscle anatomy, it should be easier for birds that normally fly with extended legs to sometimes fly with folded legs (as seen in shorebirds) rather than the other way around. This idea remains to be tested.

Now returning to the mystery, what about Great Snipes, do they fly with legs folded? Unfortunately, the authors found no evidence of this. However, Great Snipes are rarely seen and when they are seen, it is difficult to see leg posture in flight. The authors did find the behaviour in many other species, including the Common Snipe, a close relative to the Great Snipe. They therefore conclude that flying with folded legs may well occur in Great Snipes, but proof remains to be found.

Although the authors did not entirely solve their mystery, they gathered a lot of previously unknown information. Their study indicates shorebirds do sometimes fly with folded legs and that reducing heat loss at low temperatures is a likely reason for this behaviour, but not the only one. Aerodynamics and leg muscle fatigue may also play a role. The research generated both new knowledge and further questions.

This is the essence of fundamental research, which often lays the foundations for innovation. Examples abound, but here is a bird-related one. In the 1970s, researchers were studying retroviruses in chickens. When they found something puzzling, and started searching for answers, they were not looking to revolutionize the field of cancer genetics. Yet their discovery that the cancer-causing genes – oncogenes – in chicken retroviruses were actually derived from the host genome was a breakthrough. Oncogenes were now relevant for all animals, including humans. Later, researchers studying oncogenes in humans encountered a particular gene – HER2 – that was associated with aggressive forms of breast cancer. This gene became the target of research that eventually led to drugs like Herceptin for breast cancer and Gleevec for chronic myelogenous leukemia. For decades, different teams of researchers found mysteries and built upon the work of others to solve them. The culmination of all of these projects – which began with research on retroviruses in chickens⁵ – gave us life-saving drugs.

Especially in our current era with so many pressing problems to solve, it is important to remember that the best way to fuel innovation is to support fundamental research.

Stay curious.

PDF is available for download here: https://www.waderstudygroup.org/article/13808/

1. Lindström, A & T. Alerstam. 2020. Waders flying with folded legs – which species, when, where and why? Wader Study 127(2): 113-120.
2. Bäckman, J., A. Andersson, L. Pedersen, S. Sjöberg, A. P. Tøttrup & T. Alerstam. 2017. Actogram analysis of free-flying migratory birds: new perspectives based on acceleration logging Journal of Comparative Physiology A203: 543–564. https://doi.org/10.1007/s00359-017-1165-9
3. Ryeland, J., M.A. Weston & M.R.E. Symonds. 2019. Leg length and temperature determine the use of unipedal roosting in birds. Journal of Avian Biology 50(5): e02008. https://doi.org/10.1111/jav.020
4. Walker, A.M. & R.A. Meyers. 2019. The anatomy and histochemistry of flight hindlimb posture in birds. II. The flexed hindlimb posture of perching birds. Journal of Anatomy 234: 668–678.
5. Sawyers, C. L. 2019. Herceptin: A First Assault on Oncogenes that Launched a Revolution. Cell 179: 8-12. https://doi.org/10.1016/j.cell.2019.08.027