Study finds bird wings are not optimally shaped for flight

Even the albatross, among nature’s most celebrated fliers, does not have wings ideally shaped for its epic migrations.

New research suggests that evolution has left most bird species flying with wings that are less than perfect. The finding could reveal something fundamental about the way evolution works.

The study was led by Benton Walters, a doctoral researcher at the University of Bristol’s School of Earth Sciences.

Testing an evolutionary assumption

Evolutionary biology has long held that animals tend to evolve towards the best possible version of themselves.

Given enough time, natural selection is thought to refine a wing, fin or limb until it is close to ideal for the job it performs.

The researchers decided to put that assumption to a direct test, using birds.

They employed a technique known as theoretical morphospace. Instead of beginning with real birds and assessing their wings, the team first created an extensive grid of every wing shape that could theoretically occur in nature, including many shapes no bird has ever possessed.

Mapping bird wing performance

The researchers then assessed the aerodynamic performance of each theoretical shape. The result was effectively a map of wing performance, with peaks marking optimal shapes and valleys indicating poor performers.

Once they had mapped this performance landscape, they plotted the real birds onto it – all 1,139 of them – to find out where they fell.

“There has been a base assumption in evolution that animals have evolved the best possible shape for what they do, however in recent years there have been challenges to this way of thinking,” Walters said.

“Our research allowed us to test optimality and show in large groups like birds that many wing shapes are in fact sub-optimal.”

Birds without optimal wings

Most birds cluster around the middle and lower reaches of the performance map. Among them are passerines, the group encompassing most of the birds people encounter every day, from sparrows and crows to warblers.

Although these birds fly perfectly well, their shapes are a long way from optimal.

One of the more surprising underperformers was the albatross, renowned for gliding immense distances over the open ocean with scarcely a beat of its wings.

Terns also ranked low on the optimality scale. They include the Arctic tern, which undertakes one of the longest migrations of any animal on Earth, travelling from the Arctic to the Antarctic and back each year.

“Two bird groups that surprised me for not being optimal were albatrosses and terns, both famed for their long-distance globe-spanning flight,” said Walters.

“It turns out you don’t have to be optimally shaped to perform the impressive feat of migrating from Arctic to Antarctic and back every year like Arctic Terns do.”

“It turns out for many birds, including most of the ones you see every day, that good enough is good enough when it comes to flight.”

The top performers

At the opposite end of the scale, two groups stood out – an unlikely pairing.

Hummingbirds were among the strongest performers, which makes intuitive sense. Their entire existence relies on a highly specialised kind of flight: hovering with extraordinary precision while feeding from flowers. Their wings reflect that dependence.

Penguins were the real surprise. Their wings scored as optimally shaped for flight, although penguins do not fly.

Instead, they use their wings to swim, moving through water with the same mechanics that other birds use in the air.

It seems that evolution gave penguins excellent flying wings and then directed them towards the ocean.

Future research directions

The team intends to use the same method to study bats and pterosaurs, the extinct flying reptiles that ruled the skies before birds.

Wings evolved independently in birds, bats and pterosaurs, so a comparison of how well each group optimised for flight could offer fresh insight into how function shapes form across very different lineages.

Walters also wants to include early fossil birds such as Archaeopteryx in the analysis. “That will allow us to see how well these animals flew and how wing shapes changed since birds first evolved,” he said.

Broader implications of the study

The findings have a practical side too. Engineers and designers have long taken inspiration from nature, and bird wings are an obvious candidate for informing aircraft design.

“This research shows that which animals you choose to borrow from really matters when it comes to flight, but that there are potential good options for planes with bird-inspired wings,” Walters said.

The deeper finding, however, concerns evolution itself. The pressure to be perfect is not always present, it seems, and for most birds, winging it has worked just fine.