The Sciences

Why Does the Peregrine Falcon Risk Its Bullet-Like Dives to Catch Prey?

The bird is a marvel of evolution, a gifted raptor that uses its special advantages to reach dive speeds of over 300 km/hr.

When it comes to speed, humans are often effortlessly outclassed by other members of the animal kingdom. In short bursts, a cheetah can reach speeds four times greater than Usain Bolt’s. A sailfish can swim ten-times faster than the fastest human swimmer ever.

The fastest animal in the air, however, is just stupendous. It is the fastest of them all.

The peregrine falcon’s body is built to execute high-speed manoeuvres as it dives through the air to snag its prey. Tailor-made wings and feathers reduce drag by allowing the falcon to shape itself like a bullet. A large keel bone allows it to flap its wings more vigorously and generate lots of power. Nifty modifications in the eye keep its cornea from drying up in-flight. A sophisticated breathing and circulatory apparatus gives it the ability to breathe at high speeds and not tire quickly.

The bird is a marvel of evolution, a gifted raptor that uses its special advantages to reach dive speeds of over 300 km/hr. That’s three-times faster than a cheetah or a sailfish.

That said, it’s curious why the peregrine falcon engages in this remarkable aerial attack strategy at all. Scientists believe the falcon’s stoop is a payoff between surprising its prey and risking hitting a target – or the ground – at over 200 km/hr. High-speed dives also make it hard for the falcon to make mid-flight adjustments to its trajectory. In all, its a gamble of a dive, and the falcon surely has better options…

Studying the aerial dynamics of birds is not easy, especially when these dynamics involve speeds of an F1 car in full throttle, moving through air. Evaluating if an attack strategy is optimal is also complicated because each strategy is motivated by a variety of shifting factors working together.

As a result, scientists switched to using computer simulations to understand why peregrine falcons perform their boltlike dives. Their working question: does the stoop increase the odds of successfully catching prey?

According to a recently published study, the answer is ‘yes’.

When confronted with agile prey, the peregrine falcon goes for a high-altitude swoop because that’s indeed the best mode of attack. The researchers coupled the individual flight mechanics of the falcon to a navigation system, and tried to find the conditions in which a stoop attack offered unique advantages, and why.

The birds known as starlings (of the murmurations fame) have three flight trajectories: straight line flight, smooth curving and jerky turns. These courses broadly correspond to actual paths many birds are known to follow. The peregrine falcon has to design its stoop such that it catches these birds when as they’re performing one of these three moves.

In their simulations, the scientists modelled the way a falcon corrected its stoop while hurtling through the air along the laws used by guided missiles to maintain their fix on targets. This choice was inspired by the results of a study published last year, when different scientists had attached camera and GPS devices on falcons and found that their trajectory was quite similar to the path followed by visually guided missiles. In these exercises, they found that a single number called the ‘navigation constant’ was able to dictate how the falcon would compensate for its prey’s motion.

In the current study, the researchers picked one starling flight strategy at a time and ran simulations for different stoop altitudes, angles of attack and navigation constants. Across millions of simulations, they were able to distill the complex predator-prey dynamics of an actual starling-falcon system to a set of just three parameters.

At low speed, the falcon is outsmarted by the starling because the latter are better able to steer their bodies and execute tighter turns. At a high speed, however, the falcon kills it. When the starling flies in a straight line, the falcon is better off diving from low heights. As soon as the starlings begin to move around a bit, the falcon has to increase its diving height to achieve a high catch-success rate.

When the starlings start to move jerkily, in seemingly abrupt ways, the most successful strategy for the falcon is to dive from a height of almost 5,000 feet (1.5 km) – a high-power stoop in which the falcon almost reaches its maximum possible speed at the point of interception.

The high speed achieved during this death-dive gives the falcon a distinct aerodynamic advantage: the falcon is able to roll with higher acceleration and outmanoeuvre the starlings. It also has more leeway to sacrifice speed for a lifting force, allowing it to steer into tighter turns.

In sum, while the stoop puts pressure on the falcon to get its vision and flight control right, it also gives the falcon more time to respond to changes in its prey’s path. The study’s authors claim that for a fixed navigation constant (close to the empirically deduced value for missile systems), the stoop becomes the best way to attack a maneuvering prey.

Even though the simulations used in the study represented an idealised picture of an interaction between predator and prey, they have revealed previously hidden insights into why aerial predators adopt risky, challenging attack strategies when easier options seem to exist.

Apart from behavioral and environmental conditions that influence predation, there is a clear physical basis for the peregrine falcon’s dives. And these revelations have emerged from an analysis almost completely bereft of biological considerations.

Ronak Gupta recently completed his masters in fluid mechanics from the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore. He writes about all things science.