Millions of years of evolution have given birds techniques for flying with a grace that engineers have yet to match. One of these masterpieces of aerodynamics has now been explained by studying the mechanism of their alula, a structure at the bend of the wing that helps birds turn and land more gently.

Although the alula also goes under the unflattering name of “bastard wing,” the 2-6 feathers that sprout from the bird's adapted thumb are known to play a role in flight. As a feature that dates back to flying dinosaurs, its value is not in question. However, that doesn't mean we know exactly what that value is, let alone how we can apply it to our own forms of flight.

 

^{Structure of a typical bird wing with the alula in red. Credit: Muriel Gottrop via Wikimedia Commons.}

"It's not that [birds] cannot fly without the alula." says Dr. Sang-im Lee of Seoul National University. "But with the alula they seem to turn more easily."

A paper in Scientific Reports, on which Lee is lead author, notes: “The alula...is known to be used in slow flight with high angles of attack, such as landing. However, in spite of its universal presence in flying birds and the wide acceptance of stall delay as its main function, how the alula delays the stall and aids the flight of birds remains unclear.”

Lee used a combination of footage of juvenile magpies (Pica pica) in steep descent before and after moulting their alula feathers as well as wind tunnel experiments with wings from dead birds. “When the alula was present, the duration of total descending flight was shorter,” the paper reports. “Our results suggest that the use of alula should enable the bird to perform steeper descending flights with greater changes in body orientation.”

The wind tunnel experiments suggested that the airflow separated further back from the edge of the wing when the alula was present. Lee and co-authors attribute this to “a vortex [forming] at the tip of the alula feathers.” This, in turn, created vortices downstream.

The paper admits that it is still not clear how the alula suppresses flow separation at steep angles, although it provides a possible explanation. Nevertheless, “The alula-tip vortex is the key mechanism for lift enhancement and stall delay,” the authors conclude.

"Nature is full of vortices, and the animals and plants use them wisely. The role of the alula in avian flight is just one example," says co-author Dr. Haecheon Choi.

^{The vortices that appear over the alula change the airflow over the wing, allowing birds to land steeply without stalling. Credit Lee et al., Scientific Reports.}