The evolution of the beak was a seminal step for modern birds. ~ evolutionary zoologist Arkhat Abzhanov
With forelimbs for flight and hind limbs for walking, birds must rely upon their beaks for manipulative tasks. In effect, a bird’s neck is like an arm, and its beak the equivalent of a hand.
To facilitate beak use, birds have exceptionally long, flexible necks: assembled from numerous parts, supported by a complex array of muscles, tendons, and ligaments. Bird necks are like those of the dinosaurs from which they descended.
Birds often use rapid movements to capture prey. A heavy head would be a strain on the neck and spinal cord.
Dinosaurs had lightweight heads. Birds extend this trend to an extreme. The size of bird brains corresponds with the physiological requirement of apt manipulability of the beak. In other words, avian brain size has nothing to do with cognitive ability. Many bright birds have tiny brains. (Where brains are resident in organisms, they are mere artifacts of the intelligence system, which is energy-based. Physicality is always a crude correlate of coherent energy forms.)
The bones in a bird’s skull are mostly paper thin, providing little protection. Even the jaws are lightweight, despite their crucial duties.
The shape of a beak tells a poignant story of each bird’s evolution and survival. ~ American ornithologist Noah Strycker
The earliest birds had teeth specialized to their diet, but they economically lost their teeth early on. Jaw muscles moved closer to the body core.
Birds use the beak for literally everything. They evolved a versatile tool not just for getting food, but also to accomplish many other tasks. ~ Spanish paleobiologist Jesús Marugán-Lobón
Of all avian traits, the beak appears the most flexible in adaptivity. Though the bill often contributes to it owner’s repertoire of displays and communiques, its shape reflects feeding habits.
In birds of prey such as eagles and falcons, the shapes of the skulls change in a predictable way. The shape of the beak is linked to the shape of the skull, and these birds can’t change one without changing the other. Being able to break this constraint — letting the beak evolve independently from the braincase, may have been a key factor in enabling the rapid and explosive evolution of the thousands of species of songbirds. ~ English evolutionary ornithologist Jen Bright
The finches of the Galápagos Islands were seminal in Darwin’s speculations on evolution. The islands undergo cyclical wetter and drier climates. The seeds which the finches feed upon get larger in moist times and shrivel during aridity. Via epigenetic tailoring, finch beak lengths change to better eat the extant seeds.
The connection between beak shapes and feeding ecology in birds is weaker and more complex than has been historically expected. While there is definitely a relationship there, many species with similarly shaped beaks forage in entirely different ways and on entirely different kinds of food. ~ Spanish paleobiologist Guillermo Navalón
Bird bills vary vastly in size, shape, and strength. Among other tasks, beaks are adapted for tearing meat (hawks), grasping fish (terns), cracking seeds (finches) probing crevices (woodpeckers), sussing what is in the sand (sandpipers), and straining microscopic food from mud (flamingos).
Herons, cranes, and storks have simple, elongated beaks for picking up small prey, but bill length is important if a bird is to hold struggling prey away from the eyes, which must be safeguarded.
Some hummingbird beaks are adapted both for nectar collection efficiency and fighting.
Instead of feeding on a particular flower shape very well, some birds try to exclude everybody from a patch of flowers, even though they can’t feed as well on them as hummingbirds without bill weapons. If you are good enough at keeping your competitors away, then it doesn’t matter how well you use the resources in the flowers you are defending – you have them all to yourself. ~ Columbian evolutionary zoologist Alejandro Rico-Guevara
Even the most robust beaks have sophisticated structures and specializations. Near the tip are spaces for tiny taste organs and other sensory receptors which help sort out edible from inedible items. These must be kept clear. Birds frequently trim their bills by wiping the edges against some hard object.
In most birds, the horny beak is no more complicated than a pair of chopsticks. Like chopsticks, the beak is useful only because it is manipulated by a sophisticated “hand.” Whereas mammal jaws have 2 robust bones that work against each other, birds have 9 or more small bones that sometimes work together and sometimes in opposition.
Special hinges allow the upper jaw to rotate against the skull, and the arms of the lower jaw can flex outward near their midpoints. Both movements increase gape and enable a bird to cope with inconveniently shaped foods. Some bones in the anterior jaw are not attached to muscles, instead acting as levers or pushrods, bound by ligaments to bones further back in the mouth that are muscle-bound. The working of a bird’s jaw has more in common with the shuttling of a loom than with the grip of the human hand.
Bills are usually dull-colored, although some birds have colorful beaks which are used in courtship displays. Even closely related birds may have quite different beaks.
Birds have a very non-mammalian tongue, in that it actively handles food. A bird’s tongue is neither soft nor fleshy: it has its own internal skeleton, termed the hyoid apparatus. Birds employ their tongues to secure, manipulate, taste, and swallow food. If the avian jaw functions as fingers, the tongue is the thumb. Like beaks, tongues vary in length and shape.
Until the early 20th century, birds were thought to lack taste buds. Our taste buds are on our tongues. Avian taste buds are on the base of the tongue and on the roof or floor of the mouth. Whereas humans have over 10,000 taste buds, birds may have anywhere from 2 dozen (chickens) to a few hundred (parrots). Yet birds differentiate tastes quite well, with sensitivities corresponding with diet. Calidrids, a group of shorebirds, recognize where worms have been crawling in sand. Hummingbirds can discriminate among sugars and their concentrations.
Even the most robustly jawed birds can only initiate food processing in their mouths. Some species deftly crack shells off seeds, or tear chunks from prey. Most merely can orient their food intake for comfortable swallowing. Chewing is out of the question.
Even the saliva produced by some birds lack digestive enzymes. Both chemical and mechanical reduction to digestion are delayed until the food enters the muscular foregut, deep within the body cavity.
Even in the gut, birds, like their evolutionary predecessors, lack any grinding surface. Hence birds swallow grit or small pebbles for mechanical reduction. These digestive aids may not be lighter than a set of teeth, but they are in body core, not in the head.