The nightjar needs to be able to fly both long distances and at lower speeds.
Photo: Urban Rundström.
Evolution does not produce organisms that are perfect at everything. Natural selection acts on inherited variation, and an improvement in one function can come at a cost to another. A bird that must both migrate over long distances and catch insects in slow, manoeuvrable flight faces competing aerodynamic demands. The European nightjar, Caprimulgus europaeus, provides a revealing example of how these demands can produce a workable compromise.
As described in a news release from Lund University, the nightjar travels between Europe and southern Africa, yet uses the same wings to pursue flying insects at low speeds. Efficient travelling and slow-speed hunting do not necessarily favour identical wing shapes. Features that help the bird remain airborne while moving slowly can impose an aerodynamic penalty when it flies faster.
The research, published in PLOS Biology, investigated this compromise by measuring the airflow behind nightjars flying in a wind tunnel. At cruising speeds, the researchers detected an unusual double wingtip vortex associated with less efficient lift generation. They interpret this as a cost of the bird’s broad, slotted wingtips: a shape likely to help generate lift during slow flight and hovering, but which produces a less favourable distribution of lift at higher speeds.
This does not mean that nightjars are unsuccessful migrants, or that flying faster necessarily makes each kilometre more expensive. The finding concerns a particular component of aerodynamic performance. A wing can carry a bird successfully across continents while still paying a measurable price for features useful when hunting.
It would also overstate the case to present this as a decisive disproof of intelligent design, although there are good reasons to believe it refutes the idea. Human engineers encounter trade-offs too, and a compromise need not be a poor solution to a demanding combination of tasks. What the research illustrates particularly well is why evolution need not maximise every individual performance measure. Natural selection favours variants through their effects on survival and reproduction under prevailing conditions; it has no foresight and no separate ambition to produce the most efficient possible cruising wing.
The nightjar’s wing is therefore better understood as an evolved balance of competing requirements than as an example of perfection in every detail. It can be less than ideal for one task while remaining highly effective across the bird’s whole way of life. That is a more informative explanation than simply admiring its abilities and attributing them to a designer: it identifies the benefits, measures the costs, and explains why the two occur together.
This also illustrates the application of Occam’s razor: we should not add assumptions to an explanation unless they improve its explanatory power. Evolution provides a coherent, testable framework for understanding the nightjar’s combination of adaptations and aerodynamic trade-offs. Invoking a supernatural designer adds an unverified entity without specifying its methods, constraints or intentions, or explaining why it would produce this particular compromise. Unless that addition yields independently testable predictions, it contributes no explanatory advantage. The evolutionary account is therefore the more parsimonious: it explains the findings without adding a designer whose involvement remains an unsupported assertion.
Occam’s Razor: When Does an Extra Explanation Explain Anything? Occam’s razor, also called the principle of parsimony, is a guide to choosing between competing explanations. It is commonly associated with the medieval philosopher William of Ockham. Its central message is that we should not introduce additional assumptions or entities unless they do useful explanatory work.From the Lund University news release:
The principle is often summarised as “the simplest explanation is usually best”, but that can be misleading. A simple explanation that fails to account for the evidence is inferior to a more complicated explanation that succeeds. The relevant comparison is between explanations that account for the evidence comparably well: unnecessary assumptions then count against an explanation.
How does science use it?
Scientists assess explanations by asking whether they fit the observations, are consistent with other well-supported knowledge, and generate predictions that can be tested. Parsimony helps distinguish between alternatives, but it does not replace these requirements. An additional mechanism or entity is justified when it explains otherwise unexplained observations or produces successful new predictions.
This matters because almost any explanation can be protected from contrary evidence by adding convenient qualifications. An explanation that needs a fresh excuse for every awkward observation becomes less informative, even if those excuses make it impossible to disprove.
Applying it to the nightjar’s wing
The evolutionary explanation combines established aerodynamic principles with natural selection acting on inherited variation. A wing feature that benefits slow flight may impose costs during faster flight. Selection acts on the consequences of that combination for survival and reproduction, rather than independently maximising every aspect of flight performance.
Adding “a supernatural designer intended this arrangement” does not, by itself, explain the trade-off any further. Why did the designer favour this balance? What constraints applied? What observations would distinguish deliberate intervention from evolution? Without independently supported answers, the designer is an additional assumption rather than an additional explanation and raises far more questions than its inclusion answers, for example, what evidence is there that it actually exists and how does it create or influence outcomes? So it's inclusion simply adds more unknowns and questionable assumptions to the proposed answer.
Nor does saying that a designer could have chosen any possible outcome provide strong predictive power and is nothing more than an assumption. Compatibility with whatever we happen to observe is not the same as predicting what we should observe.
What the razor does not establish
Occam’s razor does not prove that gods or designers do not exist. Nor does it establish that nature must always be simple, or that the currently preferred scientific explanation is complete. It tells us that an unsupported addition earns no explanatory credit merely because it can be attached to an otherwise adequate account.
The scientific question is therefore not simply whether a designer could be imagined, but whether invoking one explains or predicts anything that the evidence-based account does not. If it adds no such value, parsimony favours leaving it out.
Nightjar wings reveal a trade-off between hunting slowly and travelling far
The nightjar migrates thousands of kilometres, yet the same wings must also help it catch insects at low speeds. A new study from Lund University shows how these competing demands have shaped the bird’s flight performance.
The European nightjar (Caprimulgus europaeus) is a nocturnal migratory bird that travels between Europe and southern Africa each year. However, its wings are not only used for long-distance journeys. When hunting insects, the nightjar must be able to slow down, turn sharply and fly at low speeds. Researchers at Lund University have now shown how this trade-off is reflected in the bird’s flight mechanics.
Using a wind tunnel, the researchers observed nightjars flying at different speeds and measured the airflow behind them. The results show that the bird’s broad wing tips, which are likely advantageous for slow flight, come at a cost. As the bird increases speed, lift production becomes less efficient.
This runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine. Instead, the nightjar’s wings appear to be a compromise between several different demands. The bird can fly slowly and manoeuvre while hunting, but the trade-off is reduced efficiency in lift generation at higher speeds.
Associate Professor Christoffer Johansson, lead author. Department of Biology
Lund University
Lund, Sweden.
The researchers also made another unexpected discovery. The nightjar generates thrust even when its wings move upward. Such an active upstroke has previously been associated mainly with bats and insects rather than birds.
A flying nightjar.Photo: Lund University.
What’s interesting is that the same type of aerodynamic solution seems to appear in quite different animals. This suggests that the upstroke does not have to be merely a passive or lift generating part of the wingbeat. It can be actively used to generate thrust when the wings cannot be folded very much.
Associate Professor Christoffer Johansson.
The findings provide broader insight into how evolution shapes animals that must perform several different tasks. A wing does not necessarily have to be optimised for a single type of flight. Instead, it may represent a balance between long-distance migration, foraging, slow flight and advanced manoeuvrability. The same principle may also be relevant for technology. Flapping drones and other flying robots, like birds, may be limited in how much their wings can fold during each wingbeat.
The nightjar needs to be able to fly both long distances and at lower speeds.Photo: Urban Rundström.We now want to investigate how common this way of generating force is among other birds and flying animals. It could help us understand which aerodynamic principles are specific to the nightjar and which are more general. This knowledge may also inspire the development of new types of flapping aircraft.
Associate Professor Christoffer Johansson.
Publication:
The nightjar’s wing illustrates an important feature of evolution: adaptations have costs as well as benefits. Features that help sustain slow flight while hunting can reduce the efficiency of lift generation at cruising speeds. The result is neither a failure nor perfection in every department, but a functional compromise between competing demands. Natural selection can favour such a compromise because reproductive success depends on the organism’s whole way of life, not on achieving the highest possible score in one isolated measure of performance.
This finding is not, by itself, a decisive refutation of intelligent design. Designers also make compromises. The difficulty for intelligent design is explaining what its proposed designer contributes to our understanding. Evolutionary biology and aerodynamics provide mechanisms through which researchers can investigate the nightjar’s flight, measure its costs and benefits, and test explanations. Simply declaring that a designer wanted the observed result supplies none of that explanatory detail.
This is where Occam’s razor becomes relevant. Once natural processes account for the findings, adding an unseen designer with unspecified intentions and methods does not improve the explanation. It merely introduces another assertion requiring evidence. A designer who can be credited with both aerodynamic efficiency and its limitations, whatever researchers discover, offers little guidance about what they should expect to find. It's inclusion thus serves only to satisfy the emotional need to include one and the political objective of those who seek to elevate religious superstition above science.
The scientific interest lies in discovering how the compromise works, what it costs, and whether similar constraints produce comparable solutions in other flying animals. Those are productive questions because evidence can answer them. The nightjar needs no exemption from physics and no appeal to supernatural craftsmanship: its remarkable abilities, together with their limitations, make sense within the evolutionary framework.
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