Flight and Dive Secrets: Falcon Nostrils and Gannets' Nostril-less Mystery

Image credit: Yale Peabody Museum / Curious Sengi.

Nothing has become a more overdone trope of human desire than bird flight out of all the animal world abilities that people have envied and subdued.  We have broken the elements of aerodynamics into something we can quantify, calculate, and model in our quest to comprehend flight.  It is clear that we have had great success with this strategy.  Consider how a Boeing 747 jumbo plane can transport people from America's East to West Coast in just over six hours.  Wagon-driving pioneers would have needed months of perilous travel through woods, endless plains, deserts, snowy mountains, more deserts, and more mountains to complete the same distance.

We often look back at living organisms and try to uncover features of evolutionary design that seem perfectly suited to specific kinds of existence because of our highly constructed perspective on flight.  Let's compare the nostrils of the Peregrine Falcon (Falco peregrinus) and gannets (Family Sulidae), two extremely distinct bird species.

Image credit: Yale Peabody Museum / Curious Sengi.

The Peregrine Falcon is a little North American raptor well known for its amazing, swift dives.  The average reported speed for these dives, or stoops is 200 mph (320 km/h).  As a result, the peregrine falcon is one of the planet's swiftest animals, "dropping out of the sky like a feathered bullet" (Hagler, 2012).  Many adaptations are necessary for such speeds, including some less noticeable ones like rerouting airflow into the nose for breathing.

It is sometimes stated that the lungs would explode at the power of 200 mph air entering the nose.  There are bony tubercles in the nares that function as baffles to safely regulate the entry of air into the respiratory system in order to stop this from happening.  According to these frequently cited accounts, Peregrine Falcon's nostrils actually served as an inspiration for the design of inlet cones for supersonic jet engines.

Image credit: Yale Peabody Museum / Curious Sengi.

What does that interesting fact actually signify, though?

First of all, going at fast speeds won't blow your lungs out.  The exact opposite.  Here, it's important to keep in mind two physical principles:  In accordance with (1) the Bernoulli Effect, which lowers air pressure as airspeed increases, and (2) the direction that is energetically preferred, high to low, respectively.  The Peregrine Falcon will experience a decline in air pressure as it achieves its top speed during the stoop due to the increasing air speed it encounters.  Inhalation depends on external air that is under relatively high pressure flowing into the low-pressure portion of the lungs.

The falcon will eventually find it very challenging to breathe when the air pressure inside and outside of its lungs approaches equilibrium.  Consider how difficult it is to breathe while riding a speeding boat, facing a strong wind, or thrusting your head out a car window on a freeway.  Because the falcon's nose has bony tubercles, airflow is slowed, air pressure is raised, and air can be pulled into the body.  It appears to be a clever evolutionary adaption for extremely fast flight.

Image credit: Yale Peabody Museum / Curious Sengi.

When we look at the evolutionary distribution of narial bony tubercles, this neat tale is somewhat thrown off.  All members of the Family Falconidae, which comprises raptors with a broad range of sizes, forms, and flight prowess, share this morphological trait with the Peregrine Falcon.  The Peregrine Falcon, which knocks its avian prey out of the air, represents one extreme of the spectrum.  The caracaras are on the opposite side, and they soar slowly while looking for carrion just like vultures. 

The remaining puzzle is why all Falconidae members have these tubercles if the Peregrine Falcon's nostrils are adapted for breathing at high flight speeds.  It's likely that all members of the Falconidae subfamily still have narial tubercles since this trait was present in their common ancestor.  It's possible that the tubercles evolved to perform a distinct function in the Peregrine Falcon, such as monitoring airspeed or temperature.  The implication is that nothing is known about the bony tubercles of the nostril in Falconidae, including their origin and potential adaptive function.

Image credit: Yale Peabody Museum / Curious Sengi.

Perhaps because we observed how engineers approached the challenge of controlling air intake in jet engines in a manner that was remarkably similar, we felt confident in explaining the function of the Peregrine Falcon's peculiar nostrils.  With an ever-improving understanding and use of rocketry, military aircraft broke more speed records in the years following World War II, but they were only able to fly so fast before their engines would choke and stall.  It was soon found that the airflow in the jet engine was being redirected away from the cylinder, carrying with it the oxygen required for combustion. 

The above-described flow from high to low pressure and the Bernoulli Effect both contribute to this issue.  Cone-shaped devices placed in the engine's inlet cause shockwaves to be produced, which slows the airflow and permits the engines to continue operating.  The development of the intake cone allowed for supersonic flying.  Chuck Yeager achieved Mach 1, which is a staggering 768 mph (1235 km/h) at sea level, in 1947 while piloting a Bell X-1 experimental aircraft.

Image credit: Phillip R. Hays

Another neat example of how Nature directly influenced engineering is the similarities between the architecture of the inlet cone in supersonic jet engines and the bony tubercle in Peregrine Falcons' nostrils.  Although I was unable to locate any literature that demonstrated experimental research on the aerodynamics of falcon nostrils had been done, this does not rule out the potential that a notion was spurred on by a chance observation.

Gannets are another swift flyer that must contend with strong physical pressures.  In order to pierce the water's surface and catch fish, these seabirds are known to plunge dive from a height of around 100 feet (30 meters). To do this, they draw their wings back and twist their bodies into a tight, streamlined shape.  Gannets have been known to travel at speeds of up to 54 mph (86.4 km/h, or 24 m/s) at the time of impact with the water. 

mage credit: Wikimedia Commons.

(Macabre statistics from 169 suicides by jumping off the Golden Gate Bridge in San Francisco have been gathered.  It was estimated that the impact velocity would be around 33 m/s.  Nearly all leaps resulted in death, with the majority of deaths coming from the hit itself.) 

Gannets, like the Peregrine Falcon, have developed a variety of traits that enable them to deal with such rigorous hunting techniques.  How does the gannet dive without getting water up its nose, as many human swimmers have discovered?

It is thought that gannets completely lose their external nares to get around the issue.

Similar to many other bird species, gannet nostrils form as embryos in the egg, with an epithelial tissue plug sealing the nose apertures and immediate vestibular chamber. 

Image credit: Yale Peabody Museum / Curious Sengi.

This stopper is still present in gannets even though it disintegrates a little bit later in development to open up the nares.  The external nares of gannets eventually get covered in bone and the keratinous sheath of the beak, known as the rhamphotheca.  Interestingly, gannets still have highly developed olfactory structures even though their nostrils are entirely closed off and there is no airflow through their nasal canal.

Why don't gannets have nostrils?  Secondary external nares, or compensating nostrils, were first identified by Macdonald (1960). They are generated by a gap at the mouth corner where the upper beak overhangs the lower.  The jugal bone, which is similar to the human cheekbone and makes up this overhang, is covered by a movable keratin plate.  

Image credit: Yale Peabody Museum / Curious Sengi.

The external pressure of the water when diving will likely cause these two parts of the skull, which are flimsily attached to the rest of the skull, to collapse against the sides of the beak, passively sealing up these secondary external nares.

An excellent defense against water forcibly entering the nose during plunge diving and perhaps inflicting damage or water entering the respiratory system appears to be the absolute lack of nostrils in gannets.  

However, Macdonald discovered some intriguing trends in other diving birds that were unconnected to them.  The Phalacrocoracidae family of cormorants dive, but from the water's surface rather than from the air.  Cormorant nostrils are small and virtually entirely covered, despite their more delicate entry into the water.  

Image credit: Yale Peabody Museum / Curious Sengi.

The Brown Pelican (Pelecanus occidentalis), on the other hand, is a large plunge diver with open nostrils.  A flap of skin covers the nostrils, which may push up against the nostril and seal it like a valve when subjected to water pressure from the outside.

It would be neglecting some of the subtleties of the tale to claim that plunge diving is directly related to the total narial occlusion in gannets. 

The cormorant's nearly complete loss of nostrils could be explained as a holdover from a species that once dived to great depths.  Or it might have changed for some other cause completely. 

Image credit: Yale Peabody Museum / Curious Sengi.

There may be an adaptive benefit to not getting water up your nose when diving, according to the Brown Pelican's alternate mechanism for sealing its nares.  Additionally, the use of a different approach to the same issue implies that there are drawbacks to losing one's nostrils.

For instance, seabirds require salt-secreting glands to help them eliminate the extra salt they consume from their bodies.  Normally, these glands discharge through the nose.  Gannets must expel concentrated salts from their mouths since they have small salt-secreting glands.

It is never sufficient to simply state that "structure X is perfectly adapted to serve purpose Y." When interpreting the function of anatomical structures.  

Image credit: Yale Peabody Museum / Curious Sengi.

The biological and technical design worlds are very different from one another.  We must account for the baggage and limitations brought about by evolutionary history, experimentally demonstrate that a certain structure does really serve a purpose that improves performance, and keep in mind that there are trade-offs.