The falcon's stoop has fascinated birdwatchers and scientists alike. “Detailed descriptions of the peregrine’s dive date back centuries,” said Robin Mills, a behavioral ecologist at the University of Groningen in the Netherlands.
More recently, researchers have studied how the birds achieve superlative speeds. The birds fold their wings to the side, a pose that gives their bodies the aerodynamics of a bullet. Their feathers flare up like fins to fine-tune their descent: Scientists who have strapped GPS trackers and cameras to captive peregrine falcons report that the birds steer themselves by the same principles missiles use to intercept targets.
Missile guidance laws explain peregrine falcon trajectories “surprisingly well,” Mills said. “Still, this research could not explain why falcons reach extreme speeds.” In other words, why did these birds have to stoop at all? A sudden attack adds the element of surprise, in theory, but no other ambush predators travel as fast as the falcon.
To answer that question, Mills and his colleagues created a computer simulation of peregrine falcons attacking starlings, a common type of prey. “The simulation incorporated the aerodynamics of bird flight, how birds flap and tuck their wings” and the missile-targeting laws as well as how falcons perceive their prey, he said.
At a straight flap, a falcon can outrace a starling. But the prey, while traveling slower, is more agile. “The best escape strategy for the starling is to perform jinking maneuvers, to zigzag constantly with well-timed hard turns to the left and right,” Mills said.
Proponents of the surprise-attack theory assumed that high-speed descents had a drawback, he said. “They thought it decreased precision of interception, and that the high speed made it harder for the falcon to follow sharply turning prey.”
After running the falcon simulations, however, Mills and his colleagues concluded the opposite was true. Falcons were most able to follow a starling's jinks during the fastest stoops, per their study in the journal PLOS Computational Biology.
“Contrary to what was previously believed, we showed that the high speed reached in stooping is beneficial precisely because it enables the production of greater excess aerodynamic force for maneuvering,” Mills said. For instance, with correctly folded wings, a stooping falcon could accelerate laterally at 15 G. That's incredible — even million-dollar supercars with lead-foot drivers can only accelerate laterally at a sixth of that.
These simulations revealed the nature of the bird as hunter: Not a dumb meteorite screaming from the skies but a skilled navigator that makes split-second adjustments. The stoop is a specialist's technique, and the falcons are uniquely adapted for it, down to the extra membrane that shields their eyes on the steep descent.
As a species, the peregrine falcon is on the updraft, with stable populations where it was once wiped out. After the ban on the insecticide DDT, which thinned its eggshells, the falcons began to recover in the Eastern United States and elsewhere. A few dozen breeding pairs live in cities like New York, where the birds, adapted to life near cliffs, chase pigeons between tall buildings.
Mills said he was confident the simulation captured the main mechanisms for the predator's success, but he plans to continue studying the falcon. “I’m sure there are many details still left to explore,” he said, and will update the model based on wind tunnel tests of the birds' wings.