Physics of Avian Beak Design

Table of Contents

Introduction

Many are familiar with the idea that birds originally evolved from dinosaurs, given their common feature of plumage. For these prehistoric beasts to emerge as the animals that swim, waddle and fly today, there was a great deal of evolution that had to take place. One of the most notable of these divergences is the progression away from the toothed snout of the dinosaurs to the wide variety of beaks borne by birds.

Teeth did not simply disappear from one generation of birds to the next, as evolution is an incredibly long process. In fact, the Archaeopteryx (Fig.1) – widely considered to be the first bird – still had all its teeth (Proctor and Lynch, 1993). As generations passed by and speciation occurred, birds kept their teeth for quite some time. Well after speciation, bird species began experiencing partial dentition reduction and edentulism – or toothlessness –independently from one another. Thus, the appearance of the beak – as it is known across contemporary birds around the world – is an example of convergent evolution. As teeth disappeared, a sheet known as rhamphotheca, composed mainly of keratin, began taking their place on the outside of the beak of many modern toothless birds. In some species, this process started at the back of the snout, while in others, it took place at the front (Louchart and Viriot, 2011).

Fig. 1 Artistic representation of an Archeopteryx. [Adapted from Bridgeman Images, n.d. Courtesy of Marshall, 2019]

The loss of teeth may seem like a disadvantage when it comes to feeding. However, when paired with a bird’s ability to move their upper and lower jaws separately, it allows them to use their beak with higher versatility. For example, birds also use their beaks to clean and straighten their feathers, to build nests, to adjust footing, to transport objects and materials, to dig or pry or probe, and in defense – not to mention the uses related to communication and mating rituals. This is more than most animals accomplish with their mouths.

Impressively, this multifunctionality does not hinder their feeding abilities, as birds do not use their beaks to traditionally “chew” their food. Many birds will use their beak to tear meals into smaller pieces, but the digestion process does not really begin until later. Instead, they hold open their jaw, and by extension, their esophagus, to swallow their food. It then passes to a part of their stomachs known as the gizzard where it is crushed (Reilly et al., 2001). Thus, they compensate for their lower chewing abilities through a unique digestion process. Their beak allows for a variety of tasks without drawbacks to the mouth’s main purpose of consumption. It is a different system than most, but a highly effective one.

For birds to develop successful flight, they needed a lightweight build. This led to theories proposing that the reason so many bird species lost their teeth was to become less heavy. However, certain discoveries refuted these theories (Yang and Sander, 2018). That being said, their lack of teeth certainly did reduce their total mass, consequently increasing their ability to fly (Fig. 2).

Fig. 2 Effects of evolutionary processes linked to teeth loss in birds. [Adapted from Louchart and Viriot, 2011]

Edentulism and the development of the rhamphotheca completely changed the way that birds obtain and digest their food, while also indirectly contributing to flight. These evolutionary adaptations, along with others, have led to the current incredibly diverse array of beaks. These vary vastly in size, shape, and color, whereas other animals’ jaws and snouts remain highly similar, as their teeth prevent them from developing certain shapes and curvatures (Louchart and Viriot, 2011).

Today’s different beaks evolved to fit specific niches. Over time, a particular design will have proved advantageous to a distinct habitat and/or lifestyle, and its helpful traits will have been amplified through eliminating unnecessary or compromising features. To shine some light on the intricacies of these structure-function relationships, this paper presents a portfolio of eight bird beaks, comparing them to man-made devices that utilize similar principles. The selected birds for these case studies are the woodpecker, parrot, spoonbill, probing birds, granivorous finches, crossbill, seagull, hummingbird, duck, and toucan. Each has been associated with a tool the writers of this report think best communicates the principles in action. These comparisons aim to demonstrate the diversity of beaks, their multifunctionality and the ingenuity of nature.

Woodpecker – like a Chisel

Woodpeckers are a notable example of impressive bioengineering design (Wang et al., 2011). They drill (or drum) on tree trunks 500 to 600 times a day, 18 to 22 times per second (Oda et al., 2006). Drumming is virtually involved in every part of their lifestyle; its purposes include obtaining grubs from inside the bark, maintaining territory and displaying sexuality. They also use it as an alternative to chirping (Oda et al., 2006). Upon drumming, woodpeckers experience a deceleration of 1000 g on impact with wood (Wang et al., 2011). To put it into perspective, the human brain is injured at accelerations of around one fifth to one half of those experienced in woodpecker drilling (Gibson, 2006).

In a comparative study of the mechanics and microstructure of the skull and beak between the woodpecker and the lark (a bird that does not perform drumming), researchers found no significant differences in the ultimate strength of the beaks. They did however find variations in their microstructures (Fig. 3 and 4). The woodpecker’s beak has more rod-like structures of greater thickness and fewer trabeculae — which are small interconnecting, beam-like structures (Davis, n.d.) — than the lark (Fig. 3 and 4), which may lead to larger deformation of the beak during impact (Wang et al., 2011). In this case, deformation is a good thing – as some part of the bird is bound to be affected by the shocks, so a design that directs that stress to non-vitals is crucial. Here, the beak that is solid enough to drill through wood is also built to cushion. Its bone has adapted to have a higher ignition loss — the mass lost when heated — than that of the lark, implying greater deformation under impact stress. Because of this beak and beak bone’s microstructures, they can partially absorb impacts, decreasing the shock transmitted to the brain case (Wang et al., 2011). Therefore, the beak and beak bone perform the function of protecting the woodpecker’s brain.

Fig. 3 SEM images of great spotted woodpecker beak. [Adapted from Wang et al., 2011]
Fig. 4 SEM images of lark beak. [Adapted from Wang et al., 2011]

This is still not enough though. Not to worry, drilling is the woodpecker’s expertise; it has its system figured out by now. Free-body analysis (Fig. 5) of the red-bellied woodpecker (Fig. 6) while drilling shows that compressive shocks acting on the beak lead to compressive stress in the base of the skull rather than travelling directly to the braincase and damaging the brain (Bock, 2015).

Fig. 5 Free-body diagram of the forces on the upper jaw on impact of the tip of the beak against a tree. [Adapted from Bock, 2015] The result of (Fi) is a large compression force (C) in the base of the brain case below the brain case.
Fig. 6 Adult male, red-bellied woodpecker. Photographed by Heidi Piccerelli. [Adapted from “Red-bellied Woodpecker” | National Audubon Society, n.d.]

It is not only reducing the impact forces but also redirecting them away from the area to protect, like a martial artist can defend a punch by deflection rather than blocking. A kinetic hinge, and the movement of the upper jaw that it allows, are key to this redirecting of the compressive force. The nasal-frontal hinge allows the upper beak to rotate up or down (Bock, 2015). Unchecked, the impact force between beak and tree would rotate the upper jaw downwards. Yet while the bird pecks, this downward rotation is countered by continuous contraction of the muscle M. protractor pterygoidei pulling it up. This muscle – which is located under the brain and connects to the beak through several intermediates – not only prevents the beak from rotating downwards while it contracts, but it also pulls the brain case backwards. This effectively eliminates the possibility of compressive shock to the braincase during the moments of impact (Bock, 2015). To further regulate the strategically moving parts, a frontal overhang of the brain case (seen in Fig. 5, right above the nasal-frontal hinge) acts as a “bony stop” (aided by the occipito mandibular ligaments) – preventing excessive rotation of the upper jaw (which would be caused by M. protractor pterygoidei) in the intervals of the drumming where there is no impact (Bock, 2015). Thus, the beak is allowed only limited movement in either direction. The nasal-frontal hinge allows for rotation, the impact creates downward rotation, a muscle contracts to counter and the frontal overhang prevents upward rotation from overcompensation. All these biological components allow the beak to wiggle and jiggle during the drumming in a controlled manner – dissipating the forces and keeping the brain safe.

When comparing several woodpecker species (Fig. 7), it is seen that with greater specialization for drilling (specialization decreases from (a) to (i)), the frontal overhang is more developed, the beak is straighter, more ventrodorsally compressed (ventro meaning stomach area and dorsal meaning back), and is wider at the base, providing more support for lateral components of the impact force (Bock, 2015).

Fig. 7 Skulls and beaks of some woodpeckers from most (top) to least (bottom) specialized in obtaining food from drilling into trees. [Adapted from Bock, 2015] Taxa: (a) Picoides tridactylus or arcticus; (b) Picoides villosus; (c) Sphyrapicus varius; (d) Dryocopus pileatus; (e) Melanerpes carolinus; (f) Melanerpes formicivorus; (g) Melanerpes erythrocephalus; (h) Melanerpes lewis; (i) Colaptes auratus.

These characteristics of specialization reduce the force at the frontal hinge (Fa) to a minimum as the impact force (Fi) and protractor force (Fp) become more in line with each other geometrically (Fig. 5) (Bock, 2015). The woodpecker also has an extended part of the tongue called the hyoid bone. The hyoid reinforces the skull, going from the jaws to the back of the skull and attaching to the nasal cavity (as seen in Fig. 8) (Oda et al., 2006).

Fig. 8 Hyoid bone of a woodpecker. [Adapted from Oda et al., 2006] The woodpecker’s skull lies within the curved portions, being supported by the hyoid.

The Lenco spring type chipping hammer with chisel and pick head (Fig. 9) is analogous to the beak and shock absorbing system of the woodpecker. The woodpecker’s beak (Fig. 10) is straight and pointed, with a flat tip – like a chisel (Oda et al., 2006). A chisel is a tool with a sharpened edge at the end of a metal blade. It is often used with a mallet or hammer to dress, shape, or work wood, stone, or metal (Editors of Encyclopedia Britannica, “Chisel”). Both the chisel and the woodpecker beak function with a potent force hitting a small area of wood with a shape that can deform it effectively. The spring in the Lenco chisel handle works to absorb some of the force of impact so it does not have to be completely absorbed by the hand using it, making it more comfortable to use. The spring performs the same function as the hyoid, the beak, and the beak bone when they deform on impact. The joint that holds the chisel and the handle together can be compared to the occipito mandibular ligaments, M. protractor pterygoidei, and the frontal overhang of the woodpecker as they hold the beak in place while drumming. The woodpecker has an incredible ability to chip wood, just like the chisel humans have engineered. Even more fascinating though is the sophistication of its design such that it can drum so intensely and so often at no expense to its brain.

Fig. 9 Lenco spring type chipping hammer with head. [Adapted from “Lenco 9010” | HomElectrical, n.d.]
Fig. 10 Beak of a woodpecker. [Adapted from Oda et al., 2006]

Bill Tip Organ – like Technologies Utilizing Touch and Detonation

Another interesting adaptation found in some beaks is known as the bill tip organ. Birds use it in all sorts of ways – from finding food to moving up and down tree trunks. Just as people use locating technologies for all sorts of purposes, birds do too. The term ‘bill tip organ’ refers to an area near the tip of the beak that gives the animal enhanced sensory abilities (Martin, 2017). It has a dense concentration of somatosensory receptors, the most prevalent of which are Herbst corpuscles (Martin, 2017) – nerve endings found all over bird bodies that provide them with information on velocity and pressure, though bill tip organs can be made up of many types of sensory cells. The area’s specific composition of receptors, as well as their density and distribution, depends on the species and its needs (Cunningham et al., “The Anatomy of the bill Tip of Kiwi”, as cited in Martin, 2017). Following are three examples of these systems: those found in parrots, in spoonbills and in probing birds – all of which are quite different.

Parrots use theirs for enhanced tactility (Cunningham et al., “The Anatomy of the bill Tip of Kiwi”, as cited in Martin, 2017). It allows them to manipulate objects with a higher level of precision, and to use their beak as a third leg (Demery et al., 2011, as cited in Martin, 2017). In parrot beaks, the mechanoreceptors are distributed in spaced out bundles along the inside edges. Poicephalus senegalus has seven groupings of receptors on either side of its beak and one at the tip (Fig. 11).

Fig. 11 Bill tip organ in parrots. [Adapted from Martin, 2017] The clusters of Herbst corpuscles are found near the tip of the beak (circled in (a)) and along the edges, as seen in (b) and (c). The zigzags are structural and help with grip (like treading on shoes); the peripheral dots are clusters of Herbst corpuscles.

This allows parrots to be very sensorially attune to what is inside or touching their beaks. Many have diets which include a lot of nuts and seeds – often ones with thick shells. They can use their beak sensitivity to move the food around and get it into a convenient crushing position. What is more, parrots use their beak to travel on trees, feeling for branches or bark they can grip onto (Martin, 2017). This sort of data collection is used in robotic hand technologies too, which have a variety of applications, such as artificial limbs, pick and place mechanisms and military robots. The ability to assess the strength of the object being handled and adjust the applied force is crucial to the machinery’s practicality. Most use tiny sensors spread out across the device’s surface, measuring resistive forces of what it has encountered. With this information, it can adjust grip strength to ensure nothing is crushed, and nothing will be dropped (Almassri et al., 2015). In effect, the robotic hand can be more sensitive than a mother’s. Parrots need this level of feedback from their beak mechanoreceptors too. Their eyes are located on the sides of the head and the beak takes up a lot of space so the bird cannot see directly forward. Thus, as is the case with many evolutionary products, this feature is both an advantageous tool and a compensatory necessity to parrots (Martin, 2017).

Similarly, many probing birds rely heavily on their bill tip organs, though they use them for hunting. Some particularly interesting species allow the birds to detect prey without sight, smell, or touch. The phenomenon is known as ‘remote touch’ and involves detecting vibrations and/or perceiving pressure differences (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). It requires bundles of dense Herbst corpuscles embedded in the bone at the tip of the beak, which can be seen in Fig. 12 (Cunningham et al., “The Anatomy of the bill Tip of Kiwi”, as cited in Martin, 2017).

Fig. 12 Bill tip organ in kiwi and ibises. [Adapted from Martin, 2017] The pictures a to g are of the kiwi’s bill. Picture a has intact keratin, while the others show the indentations where clusters of Herbst corpuscles lie. The arrows point to the kiwi’s nostrils, which are (unusually for birds) found near its bill tip organ. The pictures (h) and (i) are of an ibis bill. The two species have similar arrangements of mechanoreceptors though they evolved them separately.

Spoonbills are a species with this ‘remote touch’ ability. As their name suggests, these birds have long beaks with wide, rounded, concave tips (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). This in itself is an interesting adaptation resembling the tool called ‘spoon’. The spoonbill wades through water, leaving its beak submerged and slightly open as it swishes it from side to side until it feels some fish come inside, then it snaps its beak shut (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). Or so scientists thought. In reality, though the motion is the same, this is not a haphazard process; the animal is actively detecting nearby prey and has been observed snapping at fish just out of its range (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). It adjusts its sweeping (refer to Mitchell, 2019 in Appendix) to catch nearby creatures. The Herbst corpuscles can detect frequencies between 100 and 1000 Hz, though they make no distinction about amplitude or velocity (Martin, 2017) ; the birds just need to know something is there, and that it is moving the way a meal does. The equivalent in human innovation would be ultrasounds or SONAR, which both utilize the pressure from sound waves to locate. Ultrasounds send out frequencies from 1 to 5 MHz by making a crystal vibrate and capture the returning waves through the same crystal. From the return time, the technology can gage the material it met and how far away it is (Freudenrich, 2001). Sonar is the equivalent to radar, but it uses longitudinal waves rather than electromagnetic ones, so it is closer to what spoonbills do. The most common version sends out sound waves through a transmitter and captures their return with a receiver. Like ultrasounds, it measures the time it took for the wave to return, which indicates how far it travelled and what it met. These devices work in the 100 to 500 Hz range (Shukla, 2017). So neither is quite like the spoonbill (who can receive without emitting and utilizes a different frequency range), but the general principles of detection through pressure waves bouncing off things are similar and they each do their job best. The spoonbill does not need a portrait of the fish it is craving, just an approximate size and distance, and it certainly has no use for detecting specimens thousands of miles away (Shukla, 2017).

Other species that use ‘remote touch’ include sandpipers, ibises (Fig. 13) and kiwis (Fig. 14). They hunt in different ecosystems, such as swamps, marshy areas, forest ground, bogs and sandy beaches (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). The kiwi’s strategy is to poke its beak in mud, come back up and poke it in again. It can feel the vibrations of prey such as worms, and so can locate its food without seeing or smelling (Martin, 2017; Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017).

Fig. 13 Molting Adult Glossy Ibis. Photographed by Brian Kushner. [Adapted from “Glossy Ibis” | National Audubon Society, n.d.]
Fig. 14 Brown kiwi. Photographed by Tui De Roy. [Adapted from Minden Pictures, n.d. Courtesy of White, 2010]  

Red knots, a type of sandpiper, probe similarly, but they cannot detect vibrations. Their prey rarely makes any – their main food source is mollusks. Intrigued by this, some scientists investigated it. They found that the bill tip organ sensors of red bills detect pressure variations. They will probe the mud or wet sand with their beak, swish it around a bit and pinpoint prey from the way the ground settles after being disturbed. Sure enough, when tested, the birds could tell buckets of empty, wet sand from buckets of wet sand containing mollusks (“Red knot foraging experiment” | NIOZ | YouTube, 2013). Once they know their target is near, they can pinpoint it seven to eight times more accurately than if their search was random. But, water is essential; red bills have no remote touch ability in dry sand as they sense pressure changes resulting from objects blocking water flow through the sediment. And it is purely this texture-based recognition that informs them, as it was found that they cannot distinguish mollusks from stones before bringing them to the surface (Piersma et al., 1998 as cited in Martin, 2017). Oil mining employs a similar process when they use seismic testing to locate underground petroleum. Instead of mixing the ground, they blast it with pressure waves, creating disturbances which send the returning waves back to the sensors such that they indicate where to look for oil (“Using Seismic Technologies In Oil And Gas Exploration” | Earth | EarthSky, 2013). Both methods are based on the fact that pressure wave velocity depends on the substance being travelled through, and pressure varies when the ground is disturbed so the waves will come back differently. Other birds read pressure too. Ibises vary a lot between species and occupy many habitats; their feeding strategies involve diverse combinations of pressure and vibration recognition, and the birds’ beaks reflect their diets; each has the necessary distribution of Herbst corpuscles (Cunningham et al., “Bill Morphology of Ibises”, as cited in Martin, 2017). Scientists have found positive correlations between the extent a bird relies on the receptors and their distribution along the bill. Spoonbills and some ibises, who feed exclusively from the water, have many receptors, which gives them a larger cylindrical radius of awareness. Kiwis and ibises who forage on land, have mechanoreceptors over a smaller area, as they supplement with prey they see (Martin, 2017).

Though people measure vibrations and pressure for all sorts of purposes, from communication with microphones to transport with underground sensors at intersections, spatial analysis relies more on technologies such as radars, cameras and lasers. Select birds have utilized ambient effects in their own way, and they may yet be the source of inspiration for future technologies, be it all-terrain travel or deep-water, vibration-sensitive explorations.

Granivorous Finches – like Seed Pliers

Discussions about bird beak adaptations often involve Charles Darwin’s studies, for which finches are famous. Darwin studied the variance in beak shape in these birds based on their diets. Finches with granivorous diets vary in beak shape depending on the type of seeds they ingest: small beaks are most effective at husking (removing the shell of seeds) small seeds, large beaks are most effective at husking large seeds, and medium beaks have the widest range of seed diet (Soobramoney and Perrin, 2007). The general shape of a granivorous bird’s beak is conical: wide at the base and pointed at the end (“Bird Beaks” | Fernbank Science Center’s Ornithology Web, n.d.). Unlike birds with other diets, granivorous birds’ beaks do not need to be long to reach into small areas. Instead, the requirement for the beak shape is to crush seeds, which is optimized with a wide and deep beak (Fig. 15).

Fig. 15 Java finches (granivorous). Photographed by Stephanie Young Merzel. [Adapted from Stephanie Y. Merzel’s Flickr profile, 2012]

Not only is the shape of the beak conducive to seed husking, the layered structure of the beak also contributes to its dietary habits. The outermost part of the beak – the epidermis – is itself layered: the thickest layer, found on the outside, is made up of keratinized cells; the layers below are made up of columnar epithelial cells. The next part of the beak, the dermis, does not contribute much to its resistance to the stress of seed crushing because it is composed of connective tissue through which blood vessels and nerves pass. Below this part, the bone is found. In the bone, connective tissue known as trabeculae, which helps resist stress, is most concentrated in the part of the beak that a specific bird uses to crush seeds (Genbrugge et al., 2012).

The keratinous exterior structure of bird beaks also allows it to resist stress. While bone tends to fracture and weaken the entire structure, when keratin suffers damage in one area, the rest of the keratin surface stays intact. Keratin not only resists fracture, but it also protects the beak’s structure by being replaceable. Indeed, when the dead keratinized cells wear off, the living epidermal cells below take their place and are keratinized. This continually growing structure prevents the weakening of the beak and the wearing out of the beak’s outer structure (Soons et al., 2015).

Seed pliers are made of stainless steel. They are used to pinch seeds between each side of the pliers and crush these seeds. While the pair of seed pliers and the granivorous bird’s beak both crush seeds, they go about it in two very different ways. The seed pliers are made up of two second-class levers (the output force is between the fulcrum and the input force). The bird’s beak (not just granivorous ones) uses closed kinematic chains. A tool that is comparable to the bird beak in its operation but not its purpose would be a scissor car jack which is formed by a closed kinematic chain.

A closed kinematic chain (CKC) is a system that links multiple parts in a closed loop. The movement and behavior of one part thus affect all parts through the linkages, and the entire structure changes and adapts instantly, as a unit. CKCs are now often used to describe the connections between parts of biological systems at various size scales (Levin et al., 2017). Although the bone structure of many body parts may suggest open chain systems, muscles, ligaments, and tendons must also be accounted for, which results in a closed chain. In some cases, the bones are enough to form a loop, like in bird beaks (Fig. 16). In birds, CKCs notably apply to the rotation of their upper beak and their lower beak: beaks can rotate, while also being able to move side to side (Olsen, 2019). Given the length of a bird beak, a lever system would be inefficient and would require an impossible amount of force to simply crush seeds. Closed kinematic chains allow them to crush these seeds very effectively and perform a multitude of other tasks.

Fig. 16 Elements of the mallard skull (left) and one of many resulting closed kinematic chains (right). [Adapted from Olsen, 2019]

A study shows that the bite force of granivorous finches ranges from 2.9 N to 38.4 N (van der Meij and Bout, 2006). However, only three out of the 18 studied species exerted a bite force above 10 N. For seed pliers measuring 9 cm, the distance between the input force and the fulcrum is approximately 9 cm, and the output force can be estimated to be 4.5 cm from the fulcrum. Given these dimensions, the user’s hand would only need to exert 20 N to crush the seed with a force of 40 N, which exceeds by far the majority of these granivorous finches. Therefore, based on the lever aspects of these ‘tools’ and the resulting ‘bite’ forces, the seed pliers are far more effective at crushing seeds than the average granivorous bird’s beak.

Even though seed pliers are very effective at crushing seeds, the bird’s beak is undoubtedly more impressive due to its multifunctionality: the seed pliers have a single function and should therefore be expected to perform it at the highest level. The bird beak on the other hand can crush seeds remarkably well given the many other constraints to which it is subject.

Crossbills – like a Weitlaner Retractor

Most granivorous birds have beaks as described previously, but even within this category of beaks, there exist different adaptations and variations. One such example is the beak of crossbill birds, whose lower mandible crosses over the upper one by curving (Fig. 17) to the right or to the left. This particular trait is used to reach seeds in conifer cones that are inaccessible to other birds. To access these seeds the crossbill holds the cone in its right or left claw – depending on which side its lower beak curves — such that this lower mandible is parallel to the long axis of the cone. The bird then inserts the tip of the curve mandible under a scale of the cone. The lower mandible acts as a second-class lever: the tip of the curved lower mandible acts as the fulcrum, while the head of the bird and neck of the bird input a force, and the rest of the beak lifts the scale, thus being the location for the output force. With this lever, the crossbill spreads its jaw laterally to pry up the scale and gain access to the hidden the curve mandible under a scale of the cone. The lower mandible acts as a second-class lever: the seed. The bird’s tongue then reaches for the seed and carries it to the jaw (“How Nature Works: White-winged Crossbill Feeding Technique” | Cornell Lab of Ornithology | YouTube, 2010; Krebs, 1991; Benkman, “Crossbill Foraging Behavior”).

Fig. 17 A male red crossbill. Photographed by Ken Archer. [Adapted from “Red Crossbill” | National Audubon Society, n.d.]

Typically, beak size for all birds – as well as the strength of the surrounding muscles – regulates the size and hardness of the seeds on which these birds can feed. However, since removing seeds from a cone requires more force then husking the seeds, crossbill birds’ beaks are larger and stronger than what would usually be associated with the seeds they consume (Benkman, “Adaptation to Single resources”). That being said, the ability to probe and reach for seeds is also important for crossbills, and this is achieved through longer beaks (Benkman, “Crossbill Foraging Behavior”).

Another factor greatly influences the size of seeds on which the crossbill feeds. The beak’s upper mandible has a groove on each side (Fig. 18). The one on the side opposite to the curve of the lower mandible (depends on the individual bird) is used to hold the seeds when removing them from the cone. For the seed to be as secure as possible, the fit must be good between it and the groove. Therefore, the size of the groove is a good indication of a particular crossbill’s typical seed diet. For this reason, crossbills are specialists in that they feed on the seeds of a specific conifer instead of being generalists and feeding on a variety of seeds. They can feed on different conifer species during different seasons, but the seeds of these different species are usually of a similar size that corresponds to the ideal for that crossbill species (Benkman, “Adaptation to Single resources”).

Fig. 18 The cross-section of a crossbill beak. [Adapted from Benkman, “Adaptation to Single resources”] The seed-holding grooves are shown.

Similarly to how the crossbill’s lower mandible helps it pry apart the scales of a cone, a Weitlaner retractor (Fig. 19) is used to pry and hold open an incision during surgery. The curved ends are inserted into the incision and the plier handles are squeezed to separate the sides of the incision (refer to Fox, 2015 in Appendix). Although this retractor has curved ends like the curved lower beak of the crossbill, these curves are used to grasp the sides of the incision instead of as a second-class lever. Instead, it uses two first-class levers just like any other pliers. The main function of the curved mandible and of this tool is similar: widen an opening to access what is inside. However, they vary greatly in how they achieve this goal and what they do once it is achieved: they use different levers; the curved ends are used for different reasons; and the beak is used to hold and remove the seed found inside the cone, while the Weitlaner retractor stays in place to keep the incision open.

Fig. 19 A Weitlaner retractor. [Adapted from Asma, 2017]

Seagull – like a Clamp

For those who live in coastal regions or even inland near bodies of water, seagulls are not particularly interesting. In fact, the name seagull has been adapted from the formal name common gull which perfectly describes how ordinary they seem. However, modern gulls have a very fascinating and specific facial structure, which indicates they have likely descended from dinosaurs. Specifically, a creature called Ichthyornis dispar who had many similar traits to the seabird we know today – like their long beaks and large eyes. On the other hand, it possessed a muscular jaw and strong teeth which today’s common gulls lack (Bhullar, Bhart-Anjan, as cited in Hersher, 2018). Although they do not have teeth, their large strong beaks with serrated edges allow for the birds to hunt and scavenge a multitude of different foods (Fig. 20). The serrated edges of the beak prevent their prey from escaping and aid in tearing their food into smaller, more easily digestible pieces. The serrations found on the bill help to strengthen the bird’s “grip” on their prey by increasing friction and sometimes puncturing the prey itself (McBride, 2012). Thus, a seagull’s beak is comparable to a serrated edge clamp (Fig. 21), or even more loosely a serrated knife.

Fig. 20 Adult gulls, displaying small serrations along the edges of the inner, open beaks. Photographed by David Policansky. [Adapted from “Great Black-backed Gull” | National Audubon Society, n.d.]
Fig. 21 Serrated edge tower clamp used for securing lumber and other materials. [Adapted from “Tower Clamp” | Tallman Equipment Company, n.d.]

The utensil-like properties of the seagull’s bill, however, are not its only interesting adaptation. Given their proximity to the sea, over time many marine bird species have adapted water filtration systems, allowing them to ingest salt water without really consuming all the excess salt (Hughes,1970). This vital salt secretion gland resides in the head of the gull and connects down to the tube nostril, through a duct. Salt is expelled in the form of a solution through the nasal tube and then runs to the tip of the beak where it finally exits the birds system. Parasympathetic nerve control regulates the gland’s activity and secretory behavior. The parasympathetic nervous system is in charge of sending and receiving signals related to what is known as the “rest and digest” sector of the nervous system; it is responsible for the digestion of food and the excretion of waste as well as multiple other functions. These signals act in response to high salt concentrations, so the gland will produce a sodium chloride solution which is hypertonic, drawing the solute out to be discarded (Schmidt-Nielsen, 1960). Generally, the concentration of the fluid secreted through the gland is far higher than the concentration of the urine, meaning the salt gland likely plays a larger role in osmoregulation than the kidneys of marine birds. This process is integral to the survival of many gulls who have little to no access to fresh water. Many seagulls consume most of their necessary water intake in the food they eat. Specifically, those who are lucky enough to catch plenty of small marine animals and saltwater fish (Schmidt-Nielsen, 1960). The evolution of the seagull’s salt secretion glands has proven essential to their survival, as their kidneys are not able to eliminate the copious amount of salt that they ingest. Whereas for many mammals, including humans the kidneys are the central organ in charge of osmoregulation. If one were to compare the approximate 1/10 of its own body weight worth of salt water that a gull consumes in one day to a 150-pound human, they would find the equivalent consumption to be nine full liters of seawater. Humans’ kidneys would not be sufficient in excreting such an amount of salt and the impacts would be harmful and possibly even deadly (Schmidt-Nielsen, 1960).

Hummingbirds – like a Straw

Beak variations and innovations could not be discussed without mentioning hummingbirds. These tiny flyers and their food sources are in many cases beautiful examples of coadaptation (Temeles, 1996). The hummingbird has evolved to have a very thin, long beak which it uses to reach inside a flower and drink its nectar, like a straw. Many plants have thin, elongated flowers pollinated solely by specific hummingbird species (Dalsgaard et al., 2009). In return, hummingbirds have diversified bill length and curvatures to divide flora between themselves (Temeles et al., 2010), as illustrated in Fig. 22. People use straws and similar suction tubes for different purposes, such as removing excess saliva during a dental checkup-up, evacuating mucus from assisted breathing setups (“Suctioning” | Tracheostomy Team | John Hopkins Medicine, n.d.) and getting the last tapioca pearl from one’s boba tea. All these tubes have different shapes to fit their function; it is the same for hummingbirds. Each species has developed its own trade-offs to ensure survival from a fast- metabolism sugar diet (Temeles, 1996).

Fig. 22 Body plans of hummingbirds. [Adapted from “Giant Hummingbird” | Encyclopedia Britannica, n.d.]

Birds with short, straight beaks, like in Fig. 23, have access to many flowers. They can feed more quickly and effectively as their aim is more precise (Temeles, 1996). However, they cannot access the nectar of longer flowers. Hummingbirds with long beaks (Fig. 24) can.

Fig. 23 Adult male White-eared hummingbird. Photographed by Amado Demesa. [Adapted from Amado Demesa’s Flickr profile, 2015. Courtesy of “White-eared Hummingbird” | National Audubon Society, n.d.]
Fig. 24 Sword-billed hummingbird. Photographed by Michael Woodruff. [Adapted from Michael Woodruff’s Flickr profile, 2007]

Even then though, curvature of flowers varies immensely, especially among species pollinated by hummingbirds (Temeles et al., 2010). Certain flowers have specialized themselves so that only one species can pollinate it, and in return, that species can only feed from a restricted set of plants. The more specialized a hummingbird’s beak is, the fewer options it can choose from, but the fewer competitors it has (Temeles et al., 2010). This is how straws and other tubing work as well. For example, a vacuum cleaner often comes with a set of nozzles so it can be used to access a variety of surfaces. Certain areas require the use of a specific nozzle to clean as they are more difficult to access, such as corners or spaces behind furniture, but that specialization is not desirable when the goal is to cover the whole floor.

Beak specialization is related to behavior and is not necessary for all hummingbirds. The males of species with shorter, more versatile bills are competitive and territorial – they monopolize and defend their resources. In return, their females (Fig. 25) have slightly longer beaks so they can feed from a wider selection of flowers as they might otherwise starve.

Fig. 25 Adult female Allen’s hummingbird. Photographed by Lawrence Broch. [Adapted from “Allen’s Hummingbird” | National Audubon Society, n.d.]

Species in which the males have the longer bill almost certainly reproduce with lekking rituals, where the male puts on a display and tries to win over a female. In these species, females are more dominant and therefore less specialized, so it is the males who evolve to have more reach, allowing them access to more food but reducing the efficiency of feeding. Bill curvature trends (refer to Fig. 26) are similar. A curved bill is more difficult to handle but can access flowers which evolved to only be pollinated by that shape, so birds with long, curved bills have very little competition (Temeles et al., 2010). They are found in ecosystems with more wildlife diversity and more species competing for resources. The flowers these hummingbirds visit are mostly red or orange (a spectrum hummingbirds are drawn to and not insects) and produce no odor (which draws insects). Some even have perches for the birds’ feet (Dalsgaard et al., 2009).

Fig. 26 Sexual dimorphism in bill curvature in Phaethornithinae. [Adapted from Martin, 2017] The numbers in boxes are bill curvature ratios for different species (including extinct ancestral birds). High, positive values indicate strong bill curvatures in females compared to males. Black boxes are ratios of above 30 % and arrows on branches show 5 % or higher increases or decreases from the common ancestor.

Each beak shape comes with its pros and cons, weighing agility versus reach and choice versus competition. A mechanic or a plumber must choose the right tool to withdraw or insert fluids for different problems, and nature has made sure that there are all kinds of beaks, for all sorts of flowers. To be noted, while hummingbird beaks have been described here as straw-like, they also serve to some extent as pliers. It is the “tongue that transports the liquid” (“Hummingbird Tongues in Stunning Slow Motion” | UConn | YouTube, 2015), through passive surface tension which uncurls the tongue’s rolled edges and starts drawing fluid up when it contacts liquid. Then, when the tongue is pulled up, it re-curls and the nectar is pushed up to the mouth. This process requires little effort; a hummingbird can drink even when dead if a person were to lift its tongue in and out of reach from water. The beak merely serves to grant access to the nectar by inserting the tongue and pushing the flower open a little (Yong, 2017). In addition, some male hummingbirds use their beaks to stab rivals in the throat, “utilizing their beaks as swords” (Rico-Guevara et al., 2015; “Hummingbirds Fighting” | UConn | YouTube, 2014).

Ducks – like a Filter

Another type of filtration system is observed in ducks. They can swish around water in their beaks and retain only the food. A key component that allows them to do this are lamellae – comb-like structures located at the sides of the upper and lower beak (1 and 5 in Fig. 27). Lamellae filter food from the flowing water that exits the beak. The distance between adjacent structures is referred to as the interlamellar distance and the distance between the upper beak lamellae and the lower beak lamellae is called the lamellar separation (Gurd, 2006). The size of these spaces dictates which particles are retained.

Fig. 27 (a) Inside view of upper beak. (b) Inside view of lower beak. 1) Upper beak rim with lamellae. 2) Longitudinal upper beak grooves. 3) Transversal upper beak grooves. 4) Lingual hairs and scales. 5) Lower beak rim lamellae. 6) Lingual scrapers and flexible hairs. 7) Larynx. 8) Esophagus. [Adapted from Kooloos and Zweers, 1991]

Ducks use two mechanisms of biological filters. The first, sieving, occurs when particles are too large to pass through the lamellae filter (Gurd, 2006). The other mechanism is inertial impaction, which occurs when the water flow changes direction as it passes through the filter. This has to do with the water flow velocity, which can be adjusted by the duck. In inertial impaction, if the particle is close enough to the filter when the direction of the water flow changes, its inertia will cause the particle to be held against the filter (Gurd, 2006). Different types of ducks perform these mechanisms in different proportions. Shovelers have a small interlamellar distance, and retain prey mostly by sieving, while mallards have a greater interlamellar distance, so they can retain prey that is larger than their interlamellar distance by sieving, and they retain smaller particles through inertial impaction (Gurd, 2006).

Both filtration mechanisms require water flow. Ducks create a one-way flow of water from the front of the beak to the back through the combined movements of the tongue and the upper and lower beak (Gurd, 2006). This can be referred to as a suction-pressure pump mechanism. It maintains an inflow of water (bringing in food particles) at the beak tip and a bilateral outflow along the rims at the back of the beak (Kooloos and Zweers, 1991). This results in water being expelled along the side rims of the beak, where it is filtered through the lamellae (refer to Fig. 27). After a round of filtration, ducks clean their lamellae with the brushing action of their lingual (tongue) hairs (4 and 6 in Fig. 27). Food kernels are then transported by retracting tongue scrapers (6 in Fig. 27) toward the back along the grooves of the upper beak (2 and 3 in Fig. 27) and into the opening of the pharynx (Kooloos and Zweers, 1991).

Apart from just filtering by passing water through their beaks, articulation of the upper and lower parts of the beak allows ducks to move them apart and thus increase the lamellar separation so that it exceeds their interlamellar distance. This allows them to selectively expel unwanted particles that are larger than their interlamellar distance, that would otherwise be trapped by the filter (Gurd, 2006). One of the consequences of the adjustability of the filter is that the interlamellar distance does not always set the lower limit on the size of prey retained. Moreover, small particles will always have lower retention rates, even if they are larger than the interlamellar distance, because the water pumping action results in variation of the lamellar separation during filtration, allowing more particles, large and small, to escape (Gurd, 2006).

By adjusting lamellar separation, ducks can partition prey by size, actively avoiding the less preferred particles (Gurd, 2006). From an ecological perspective, this can be a positive, as varying species can partition varying particle sizes, reducing interspecies competition and therefore allowing the environment to support more species. In general, ducks with larger interlamellar distance consume larger prey, while species with smaller interlamellar distance consume prey at greater rates than those with wide spacing (Gurd, 2006).

The filtration apparatus of the duck is analogous to vacuum filtration, a chemistry laboratory technique (Fig. 28). Normal filtration depends on gravity, while in vacuum filtration, the suction provides the pressure to drive filtration, increasing the process’ speed (“Vacuum Filtration” | Chemistry LibreTexts, 2019). As is the case with ducks and their beak’s suction pressure pump mechanism (Kooloos and Zweers, 1991), the filter in vacuum filtering collects the useful solid, while the liquid is discarded. In vacuum filtering, the filtrate is collected in the armed flask and in the case of the duck, the filtrate leaves the beak through its side rim openings. In vacuum filtering, the filtrate is collected in the armed flask, and, in the case of the duck, the filtrate leaves the beak through its side rim openings.

Fig. 28 Vacuum filtration set up. [Adapted from “Vacuum Filtration” | Chemistry LibreTexts, 2019]

Toucans – like a Radiator

Many evolutionary biologists would agree that the toucan’s bill is one of the most bewildering products of evolution due to its seemingly unnecessary size. While some believe it helps birds to gather hard-to-reach fruits, others would argue it increases their ability to fight for resources and warn predators. Charles Darwin even considered its possible role in sexual selection, suggesting larger beak size made the birds more desirable to potential mates. Granted all these theories are partially true, the primary reason for its extreme dimensions are the thermoregulation properties of the beak. Unlike humans, birds do not sweat and therefore need another method to regulate their body temperature. Especially toucans, given they live in tropical highly humid climates. No matter which method they use, all endothermic organisms regulate temperature by balancing metabolic rate and exchanging heat with its surrounding environment (Tattersall et al., 2009).

Fig. 29 (a) Exhibits the complex vascular structure of an adult toco toucan’s beak, when examined closely.(b)Provides a view of the “lightweight” bone structure of the rhamphotheca, with letters indicating different parts of the bill’s structure (c = cranium, l = lower jaw, n= nares, r = ramphoteca, t = turbinates, u = upper jaw). [Adapted from Tattersall et al., 2009]

In terms of structure, the beak amounts to one third of the length of the bird’s body, however, it only makes up approximately one twentieth of the bird’s mass (Seki et al., 2005). The reason being that the beak is made up of mostly air and lightweight, firm, foam-like material encased by layers of keratin (Fig. 29b). The “foam” itself is composed of highly compacted cells, which increases the rigidity of the beak (“Engineers Discover Why Toucan Beaks Are Models of Lightweight Strain” | University of California – San Diego | ScienceDaily, 2005). An intricate network of superficial blood vessels can also be found within the beak. This system of blood vessels is multifunctional, as it not only supports the rhamphotheca, but it also plays an integral role in regulating heat exchange (Tattersall et al., 2009) (Fig. 29a). Rhamphotheca being another term for beak which comprises the upper and lower jaw bones and their keratinized sheaths. The “sheaths” are a type of covering which encases the inner more delicate structures of the bill and are made of fibrous protein called keratin (Seki et al., 2005). The rate at which blood flows will determine the rate of energy exchange with the toucan’s surrounding environment. As blood is transported closer to the bill’s surface, the heat it carries may dissipate into the air whereas, in colder conditions, the inner biological systems of the bird will decrease blood flow to minimize heat loss.

Of the toucan species, the Toco Toucan has the largest bill, making them the ideal species of focus for many studies. For example, a study was conducted to examine the effect of environmental temperature on heat exchange. More specifically, to identify the region of the body and the beak which contributes more to heat dispersion and internal temperature regulation. The researchers hypothesized that to cool themselves, the birds’ metabolic rate must increase (Tattersall et al., 2009). However, to conserve heat in colder climates, their metabolic rate must decrease. Similarly, they inferred that most of the heat exchange would occur in the regions of exposed skin and the proximal region of the beak, given that it is closer to the body where most of the heat is stored. Infrared thermography was used to detect change in the rate of heat exchange in different sectors of the bird’s body in the specific climate each toucan was subjected to. It was found that as the temperature of their surroundings changed, the most significant change in the birds’ body temperature occurred in the birds’ backs, eyes and bill regions (Tattersall et al., 2009). It is expected that regions covered in feathers will have minimal temperature variance as the feathers act as insulation containing body heat. Nevertheless, the change in temperature of the proximal and distal regions of the beak displayed a trend confirming both that the beak is highly effective in regulating temperature and that the researchers’ hypotheses were fairly accurate (Fig. 30). The study also established that the proximal region of the beak was generally used to “dump” heat at lower ambient temperatures, while the distal region of the beak aids in managing the excess in heat load by receiving increased blood flow, once again confirming the researchers’ previously formed ideas of how the regions of the beak contribute to temperature regulation (Tattersall et al., 2009).

Fig. 30 Plots which compare the average difference in temperature between the surface temperature of the beak (Tsurf) and the ambient temperature (Ta) compared to the distance along the bill. [Adapted from Tattersall et al., 2009] The graphs are organized from top to bottom in order of increasing ambient temperatures, investigated in the experiment. Graphs on the left-hand side use different colored circles to depict differences in adult (black) vs juvenile (gray) toucans. The middle column consists of thermographic images of both adult and juvenile toucans at each of the ambient temperatures. The right-hand column displays the juvenile: adult ratio for the difference in beak surface and ambient temperatures.

Some characteristics of the toucan’s bill which increase its likeness to a traditional thermal radiator include its spaciousness, complex structure, and the fact it has little to no insulation. Moreover, human engineered car radiators circulate liquid coolant through the engine which traps and transports heat to the exterior of the car. This process is analogous to blood flow throughout the body and into the beak where heat disperses into the air. Nonetheless, a radiator one might find in their house seems to have an inverse function. They draw cool air in, which is warmed while passing over the radiator fins until it is finally expelled. As impressive as these inventions are, they cannot compare to the millions of years of evolution and natural development that have contributed to the complex beaks of the very unique toucan. Marc A. Meyers, a materials scientist and professor of aerospace engineering, analyzed toucan bills by using a computer modeling system. He claimed “that the beak is optimized to an amazing degree for high strength and very little weight”, and that “it is almost as if the toucan has a deep knowledge of mechanical engineering” (“Engineers Discover Why Toucan Beaks Are Models of Lightweight Strain” | University of California – San Diego | ScienceDaily, 2005). Many animals use their metabolic heat production to maintain a relatively constant body temperature, however, a toucan’s beak’s ability to moderate temperature is particularly impressive given it only makes up approximately 30 % to 50 % of the birds’ surface area, yet its heat loss account for anywhere from 25 % to 400 % of its metabolic heat production. In contrast, most other animal species have temperature regulating appendices which only account for 9 % to 91 % metabolic heat production, so in terms of its size, its thermoregulation properties are some of the most impressive in the entire realm of living creatures (Tattersall et al., 2009). Thus, the toucan’s intricate bill is not only beautiful, but is also a multifunctional marvel of evolution.

Conclusion  

There are so many bird species out there. Their habitats literally span the globe, from the snowy owl in the north to the penguin in Antarctica and passing through every country in between. Their diversity is to be expected, as they have been fine-tuning their strategies for survival since the time the first bird, the Archaeopteryx, roamed the earth. Still, this phenomenon is impressive. The two-legged creatures use their beaks for a multitude of tasks: feeding, preening, communication, travel, transport, defense, mating, and each does so differently. Their beaks are practical and multifunctional, even when flamboyant. Many resemble common tools such as chisels, straws, and pliers, while others are more elaborate, with temperature or salinity regulation systems. Some have inspired human innovations, like the Kingfisher’s aerodynamics; others may yet, such as the bill tip organs. Trying to understand how each works by comparing it to tools people use only begins to explain the mechanisms employed. In reality, their diversity surpasses humanity’s needs and nature has developed technologies that people have yet to harness. Waking up on a winter morning, one might think the spoon they use to eat cereal, the boots they put on — designed to grip on ice — and the high-speed train they catch on their way to work are all luxuries they can enjoy due to brilliant human minds. This is correct, but looking a little closer, one might realize that the wildlife around them — though at first glance simpler — is utilizing similar advancements. Looking closer still, there may be much to learn from the engineering involved in even biodiversity’s most mundane tasks.

Appendix

“How Nature Works: White-winged Crossbill Feeding Technique.”, 16 Apr 2010, Cornell Lab of Ornithology, YouTube, https://www.youtube.com/watch?v=1NvU8WG9bg0.  

Fox, Steven M. “Securos Surgical – Instrumentation: Weitlaner Retractor.”, 18 Dec 2015, Securos University, YouTube, https://www.youtube.com/watch?v=yTjZBj-lQ4A.    

“spoonbill foraging.”, 21 Mar 2019, Tom Mitchell, YouTube, https://www.youtube.com/watch?v=Te-8G5K88SA.  

“Red knot foraging experiment.”, 11 Mar 2013, NIOZ, YouTube, https://www.youtube.com/watch?v=W5DNGRBj9IM.  

“Hummingbirds Fighting.”, 3 Nov 2014, UConn, YouTube, https://www.youtube.com/watch?v=nR2Pf1c9vfk.  

“Hummingbird Tongues in Stunning Slow Motion.”, 19 Aug 2015, UConn, YouTube, https://www.youtube.com/watch?v=QYoYQAbPXbU.

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