Spotlight: Just fast enough

by Deborah Buehler originally published in Wader Study 131(1)

Flying takes energy—a lot of it.

The airlines are certainly making this argument, as evidenced by the price of plane tickets these days. Humans can’t fly without substantial technical assistance, yet birds fly all the time. Shorebirds, for example, fly when migrating to and from breeding grounds, when moving between feeding areas, and when displaying to attract mates during the breeding season. Given that flight is costly, it makes sense that birds might try to optimize aspects of their flights to minimize energy consumption and to maximize reward.

In this issue of Wader Study, Anders Hedenström reports on his investigation into how Common Redshanks Tringa totanus adjust their flight speed to optimize energy budgets in different ecological contexts.1 How birds modify aspects of their behaviour in the context of optimization theory has fascinated shorebird researchers for decades. Flapping flight is energetically costly and studying it has led to theoretical models, that predict optimal speeds in different circumstances.2 Hedenström’s study investigates whether wild Redshanks adjust their flight speeds in accordance with model predictions.

Anders Hedenström with equipment to track flight speed at the field site on the island of Öland in the southwestern Baltic Sea. (photo: Lotta Berg)

What makes flight costly? The mechanical power required to fly comes from the work done by muscles to overcome the pull of gravity and drag. To generate this power, muscles consume energy from stores of fat, protein, and carbohydrates originally ingested as food. Researchers have found that the relationship between the cost of flight and the speed of flight is U-shaped. In other words, flying takes the most energy during take-off and at very slow speeds, energy consumption decreases at moderate ‘cruising’ speeds, and then rises again at very fast speeds. Researchers use this ‘power curve’ to derive theoretical ‘optimal’ flight speeds in different circumstances including migration, foraging and displaying to attract mates.

During migration, cruising speed is predicted to either optimize flight speed to minimize the time spent migrating or to minimize the total energy costs of the journey. For example, if early arrival at the breeding grounds means beating competitors to the best territories, then the first strategy of time minimization might be favoured. On the other hand, if food resources are limited, a strategy that favours energy economy might be best. A way to understand this in human terms is buying a plane ticket. If you need to arrive urgently, perhaps to visit someone who is ill, you might buy a more expensive flight to get to your destination faster. However, if money is very tight and you have time to spare, the optimum strategy might be to take a cheaper flight with several stopovers. This will allow you to make the journey with the funds you have available, but you’ll arrive three or four hours later.

Birds also fly when searching for food. These foraging flights are short trips taken between local food patches. When foraging, birds usually fly faster than cruising speeds during migration, especially if the habitat provides plentiful food. How fast will depend on the rate at which the birds can lay down fat and energy reserves (fuel deposition rate). Theoretically, birds will either optimize flight speed to minimize the time spent foraging or minimize the energy expended to gather food. Perhaps there are predators in the area and the birds must forage as fast as possible before retreating to a safer place to rest. However, if food is scarce, birds might fly more slowly so that they can maximize their net energy gain by minimizing energy spent feeding. In our human analogy, foraging flight is like working to save money for the plane ticket. If you have a good job and can get long shifts, you’ll save quickly even if you need to spend a bit of money to get to work. On the other hand, if work hours are few and you have a long commute, you might skip a short shift because the pay is not worth the expense of getting there. Money saved is money earned after all.

Finally, during the mating season, some birds spend a lot of energy on intricate display flights to attract mates. Because flight is costly, these displays are considered honest signals of fitness including health, good genes, and ability to provide resources. In some species, a long and drawn-out display might be preferred. In this case, the optimal flight speed would minimize the energy expenditure per unit of time. One the other hand, if acrobatics are preferred over display length, then the expected flight speed would be faster. In our human analogy, perhaps this is money spent trying to woo someone to come with you. You might spend more to convince someone who might pay for your ticket next week than on someone who might just share the costs of the hotel and rental car.

In birds these predictions are fascinating, and though they have been validated in some species in wind tunnels,3 they are mainly theoretical for birds flying in the wild. Hedenström studied flight speed in several species but chose Redshanks for this investigation because they display a range of flight behaviours including long haul migrations in spring and autumn, ‘commutes’ between local feeding areas, and aerial displays to attract mates. He was able to observe all these behaviours in wild Redshanks on the island of Öland in the southwestern Baltic Sea.

Researcher tracking flight speeds. The infrared anemometer is measuring wind speed and direction in the background. (photo: Anders Hedenström)

Hedenström observed the birds at three study sites on a total of 81 days between April and October spread over the decade from 2012 and 2022. To measure flight speeds in the wild, he followed birds using an optical ranger finder, an ‘ornithodolite’, made from binoculars with built-in sensors for elevation angle and north, east, south or west bearing (azimuth). The ornithodolite is so named because it is used to study birds, ‘ornitho’, and because it works a little like a theodolite (an optical instrument, sometimes seen mounted on a tripod and used by surveyors to measure angles). 4 Distance from observer to bird was measured with an infrared laser. Concurrently, he measured wind speed and pressure either using an ultrasound anemometer near ground level, or the range finder to track helium filled balloons at higher altitudes.

Close up of the ornithodolite rangefinder. (photo: Anders Hedenström)

A series of time-stamped distance, elevation, and azimuth data for a bird constituted a run. Runs were classified as migratory flight, foraging flight or display flight based on flock size, time of year and flight behaviour. These were then combined with wind speed data so that airspeed could be deduced as the speed of the bird over the ground minus wind speed. These wind-corrected data yielded tracks which could be plotted on a map. Hedenström was able to analyse data from a total of 139 tracks distributed across migration (N = 84), local flights (N = 29), and display flights (N = 26).

The data collected from Redshanks in the wild was compared to predictions generated by a theoretical model scripted in the R open-source programming language.5 The model produced a U-shaped relationship between aerodynamic power and flight speed for a bird of approximately Redshank size as determined by a sample of mass and wing measurements from Redshanks.

The results indicated that airspeed differed depending on the ecological context (migration, foraging or display flight) and was influenced by flock size. During migratory flight, Redshanks flew faster than the predicted speed associated with the minimum cost of transport. This could indicate that the birds were minimizing the overall time required for migration. This is akin to opting for a faster flight which costs more rather than a slower but cheaper one. When foraging, Redshanks flew between patches of food (foraging areas) at faster airspeeds than birds during migration. This is consistent with the assumption that the birds need to save the most energy over the shortest period, just like when you work two jobs to pay for your plane ticket. Finally, when flying to perform aerial displays to attract mates, Redshanks flew at a speed predicted to use minimum power. This makes sense since the purpose of display flight is not to cover distance, but rather to spend as little energy as possible while still attracting a mate.

Though Redshanks seemed to adjust their flight speeds in accordance with theoretical predictions regarding migratory, foraging and display flight, their airspeeds in relation to flock size and climbing speed were not as expected. The birds increased their airspeed with flock size rather than decreasing it as predicted, and they climbed towards cruising migration altitude at a much lower speed than the maximum possible. These results indicate that factors other than those considered by the model might be in play and more research will be required to solve these mysteries.

Hedenström’s research shows that aerodynamic theory, and the models derived from it, are useful in predicting and understanding how birds optimize flight speeds in the wild. Though the data presented are limited to a single species and study area, they provide a tantalizing glimpse into how a seemingly simple behaviour, adjusting flight speed to balance energy budget, can be a rather complex exercise in optimization.

Budgeting to buy a plane ticket provides an analogy for understanding the physiological optimization that goes into the lives of migratory birds. However, many people fortunate enough to afford recreational air travel do not have to think about how best to budget energy for survival. This study reminds us that organisms living in the wild are constantly balancing their energy budget. Anyone who has needed to fight for survival under conditions of restricted food, water or shelter knows this intimately. There is a link between energy and flight in humans too. The number of people on the brink of starvation rose from 80 million in 2017 to 350 million in 2023 in part due to the COVID-19 pandemic, climate shocks, and ongoing conflicts, yet there is $400 trillion worth of wealth on the planet.6 The coming decades could bring unprecedented human migration as people flee from unlivable circumstances. Perhaps we can learn something about the optimal distribution of resources from birds.


1 Hedenström, A. 2024. Adaptive flight speeds in the Common Redshank Tringa totanus. Wader Study 131(1): X–X.

2 Hedenström, A. & T. Alerstam. 1995. Optimal flight speed of birds. Philosophical Transactions of the Royal Society B 348: 471–487.

3 Tobalske, B.W., T.L. Hedrick, K.P. Dial, & A.A. Biewener. 2003. Comparative power curves in bird flight. Nature 421: 363–366.

4 Pennycuick, C.J. 1982. The ornithodolite: An instrument for collecting large samples of bird speed measurements. Phil. Trans. Roy. Soc. B300: 61–73.

5 KleinHeerenbrink, M. & A. Hedenström. 2023. Tools for modelling of animal flight performance. R package version Accessed at:

6 Lederer, E.M. (2023). UN food chief: Billions needed to avert unrest, starvation. AP News. Associated Press. Accessed 13 Mar 2024.


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Featured image: (c)Global Flyway Ecology