Beaks and bills

All birds have bills (or beaks), and these exhibit so much variation in their structure that they are useful as one of the distinguishing characteristics among species. Bills are toothless, except in embryos, and are covered with a horny sheath. Many of the variations in bill form are associated with specialized adaptations to different feeding habits. In addition to their obvious role in feeding behavior, bills have a function in a number of other behaviors. Some birds use their bills in different ritualized displays, including threat displays and appeasement displays. Some bills are used for digging, for nest-building, for “sewing,” for drilling, and for many other activities.

Structure of the Bill

Anatomically, bills are a compact, modified layer of epidermal cells formed around the bony core of the upper and lower jaws, or mandibles. The bill is normally hard and thick, but it is not rigid, and different bills can be bent and twisted in different ways and to different degrees. For their size, bills are relatively light, one of the many adaptations in birds for flight. Although bills do not have teeth, adaptations in some forms have functions similar to those of teeth. Some bills have sharp edges, and these may function in cutting food, for example. There also are different specializations in the bony and muscular elements that operate in conjunction with bills to compensate for the different physical stresses imposed by feeding. It is interesting to consider where birds’ nostrils are located. In most species, they are found near the base of the upper mandible, but there are some notable exceptions. In that unusual bird, the kiwi, perhaps the only species of bird with a well-developed sense of smell, the nostrils are located at the tip of the bill. This structure allows them to use their beak to find underground insects and worms without seeing them.

Some species of birds, including puffins and pelicans, have structures or projections on the bill that are present only around the breeding season and are then shed after reproduction. The brightly colored, triangle-shaped bill of the puffin has scales on both the upper and lower mandible, which seem to serve a role during courtship.

The bill of the developing embryo possesses an interesting structure: the egg tooth. The egg tooth is a hard, calcified structure on the tip of the bill of a bird embryo that is used to help the bird crack and weaken the shell so that hatching can take place. The embryo also possesses a special muscle, the “hatching muscle,” which provides the force behind the egg tooth. After hatching, the egg tooth either drops off or is absorbed.

Variations in Bill Structure

Given the variety of foods eaten by different bird species, it is not surprising that there are so many differences in the form and size of bills. What may seem surprising, however, is the observation that closely related species may have quite different types of bills. Two good examples of this phenomenon are seen in the finches of the Galápagos Islands and in the Hawaiian honeycreepers. These examples nicely illustrate the process of adaptive radiation, by which many species evolve from a single ancestral species in adapting to new habitats. Like the Galápagos Islands, the islands of the Hawaiian chain are of volcanic origin and even farther removed from any major landmass than are the Galápagos. It is thought that the ancestral form of the modern Hawaiian honeycreeper was a nectar-feeding honeycreeper type of bird that migrated to Hawaii from South America. The birds underwent a remarkable evolution, with new species formed as a result of adaptation to new feeding opportunities. Species became different not only in the shape of their bills but also in their coloration. Species were produced that were red, black, gray, yellow, and green. Beaks varied from those that were large and heavy for crushing seeds to long, thin beaks suitable for collecting nectar from flowers.

The differences in beaks among closely related species are remarkable, but even stranger are differences in bills among individuals of the same species. Bills of three different sizes are found in a finch from Africa, the black-bellied seedcracker. The bills occur in three discrete sizes, ranging from relatively small to relatively large, and the sizes appear related to the type of seeds eaten, with large bills being better suited for dealing with hard seeds and small bills being better adapted for soft seeds. Bills of intermediate size do not fare very well with either type of seed in comparison with either the small or large bills. However, the intermediates are necessary to have available the genetic capabilities for the other two sizes of bills. This type of selection is an example of disruptive selection, which, in this case, tends to produce individuals of the two extreme bill sizes.

Differences in bill structure between males and females of the same species are also known. The huia (probably extinct; last seen in 1907) is a bird in New Zealand with several unusual characteristics, the least of which is the structure of the bills in the two sexes. As is true of bills in general, the form of the bill appears related to its function. Males have a relatively straight and sharp bill, whereas females have a thin, downward-curving bill, which is much longer than that of the male. Males use their bills to dig open the burrows of grubs living in dead wood. After the tunnels are open, females use their bills to pull out the grubs. The different structures of the bills in males and females appear related to an interesting division of labor during feeding.

Another example of interesting variation in bills is a case in which the color of the bill changes seasonally within the same individual. The sex hormones, testosterone and estrogen, are responsible for the development of secondary sex characteristics and behavior in males and females, respectively. There are striking changes in behavior during the breeding season. The bills of male and female starlings change color to yellow in the males and to red in the females due to the action of these hormones during the breeding season.

Modifications of Bills for Feeding

As different species of birds are observed feeding, one has a sense of awe at the great diversity of dietary items and the variety of bills that are specialized to eat them. Foot items include nuts, hard seeds, soft seeds, insects, larvae, roots, flowers, sap, fungi, nectar, carrion, and other types of organic matter. The specializations observed in bills are adaptations for specific types of food. In a brief discussion, it is not possible to describe the many variations seen in the size and shape of bills, but even descriptions of a few representative examples provide the basis for appreciation of the exquisite designs and functions of bills.

In carnivorous birds of prey, such as falcons and owls, the beak is used as a meat hook. The birds typically seize their prey with their feet and disarticulate the cervical vertebrae by biting into the neck. The bill has a so-called tomial tooth on each side of the upper bill just behind the curved end of the beak. There is a notch or groove on the lower mandible that fits with the tomial tooth.

Birds that catch insects in flight, such as flycatchers, have beaks that are broad and slightly hooked. Frequently, there are long bristles or feathers at the base of the bill, which facilitate the catching of the moving prey.

The bill of the crossbill shows some unusual features. The tips of the beak do not meet when they are closed. The tips are displaced to the side and move past one another as the mouth is closed. The end of the lower mandible may cross to the left or to the right of the upper mandible. This type of bill appears adapted to serve as a wedge to pry open the scales of pinecones, enabling the bird to insert its tongue and remove the seeds from inside.

Variations in bill and beak types serve various purposes. An interesting specialization is seen in the beaks of tropical fruit pigeons. These birds feed on large fruits, and their bills can open not only in the vertical plane but also in a horizontal plane, like the jaws of a snake when swallowing large prey. Another interesting variation is the toucan's beak, which serves as a natural radiator, cooling the bird as it forages for food.

Another bird with an interesting specialization is the woodpecker finch endemic to the Galápagos Islands. Traditional woodpeckers use their long tongues to extract insects from trees and plants. However, the woodpecker finch has a relatively short beak and tongue, so it uses tools like cactus spines to probe for insects.

It is evident from these examples that bills of birds have become adapted for their feeding. There are even some examples of species whose bills have become so specialized for their specific food that the number of birds is completely dependent on the availability of that food source. Beaks and bills continue providing important information about evolution and Earth’s history. An interdisciplinary team of biologists, physicists, and mathematicians from Harvard, MIT, and Imperial College London developed a mathematical cellular growth model to analyze beaks and beak fossils. The model was used to quantify each beak type’s performance, shape, and function across species and to predict the shapes and functions of beaks in prehistoric birds.

Principal Terms

Adaptive Radiation: The process by which many species evolve from a single ancestral species in adapting to new habitats

Display: Usually a visible movement or behavior used as a social signal in the context of aggression, courting, etc.

Egg Tooth: A hard, calcified structure on the tip of the bill of a bird embryo that is used to help the bird break its shell during hatching

Polymorphism: The occurrence of two or more structurally or behaviorally different individuals within a species

Sex Hormones: Hormones—androgens in males, estrogens in females—which are associated with sex characteristics and sexual behavior

Sexual Dimorphism: A difference in structure or behavior between males and females

Bibliography

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