Secret feather flaps help a falcon control its dive

A peregrine falcon’s prey probably never sees death approach. That’s because a falcon is just so fast — the fastest animal on the planet, capable of reaching speeds of more than 320 kilometers per hour during a nose-dive to attack its chosen meal. That’s faster than a lot of expensive performance cars.

Given the speeds involved in such a dive, it’s not so surprising that these attack moves have proved difficult to study. But a clever experiment with a trained falcon has found that the birds get a little unexpected help from some specialized feathers in controlling their motion. The research, led by Benjamin Ponitz at TU Bergakademie Freiberg in Germany, was published February 5 in PLOS ONE.

The researchers worked with a falconer, who trained his falcon to dive off a 60-meter-tall dam in Hellenthal, Germany. The scientists then recorded 35 flights with a stereo high-speed camera system and a couple of high-resolution digital cameras. Because all the flights took place in the same spot, the images could be easily matched by lining up features on the dam wall, such as a painting of a deer. (There was no explanation of why the dam had a painting of a deer on it.)

A falcon’s dive, the experiment revealed, could be divvied up into six phases:

1. Acceleration/diving phase (0-1.1 seconds): The bird increases its velocity to 81 kilometers per hour, covering a vertical distance of 18.55 meters.
2. Transient phase (1.1-1.3 seconds): The falcon continues its dive at a near constant speed.
3. Deceleration and flight correction (1.3-2.6 seconds): The bird slows down to 70 km/hand flattens out its path of flight.
4. Pull out phase (2.6-3.5 seconds): The falcon opens up its wings, reducing its speed by about half and readying itself for landing.
5. First landing phase (3.5-5.0 seconds): The bird travels at constant speed across the ground.
6. Second landing phase (5.0 to 6.8 seconds): The bird decelerates completely and touches down.

Falcons use the positioning of their wings to achieve higher and higher speeds, first creating a diamond shape, then assuming a tight vertical tuck and finally completely folding the wings tightly against its body. Because the bird in this experiment was diving from a relatively low height, it only reached the diamond-shape stage.

One of the falcon’s flights off the dam served as a model for a faux falcon that was tested in a wind tunnel. The wind tunnel experiments let the researchers examine how air moved across the bird’s body during its flight.

The wind tunnel research showed that there was an area on the bird’s body where the air wasn’t flowing as it did elsewhere. The researchers went back to their photographs and discovered that there were feathers in that region that popped up during the dive. At other times, the feathers laid flat. The feathers probably help the air to flow properly across the falcon’s body during its dive, the researchers say.

For researchers to determine the function of those feathers, they’d probably have to tape them down or otherwise manipulate them on a real falcon, then let the bird loose. I’m not sure that’s a great idea, though, because the worst-case scenario could be a deadly belly flop for a bird known for its spectacular dives.