Birds
The Evolutionary History of Birds
Birds are members of the class Aves, a class which is closely related to the class Reptilia (which contains the reptiles). Some taxonimists and cladists have been debating whether or not to include birds in with the class Reptilia, hence eliminating the class Aves; however, the current consensus is that birds are in their own class and related to, but distinct from, members of Reptilia. It is believed that modern birds descended from the reptile family, specifically the dinosaurs. Birds, and their closest relations, the crocodiles, are believed to be the direct descendants of dinosaurs. The fossil evidence reveals one early bird-like ancestor, Archeopteryx lithographica, that was small and had feathers similar to modern birds, but had teeth and a bony tail similar to the dinosaurs. Below is a reconstruction of Archeopteryx (left) and a photo of the fossil itself (right) from the Smithsonian Institute.
Birds are members of the class Aves, a class which is closely related to the class Reptilia (which contains the reptiles). Some taxonimists and cladists have been debating whether or not to include birds in with the class Reptilia, hence eliminating the class Aves; however, the current consensus is that birds are in their own class and related to, but distinct from, members of Reptilia. It is believed that modern birds descended from the reptile family, specifically the dinosaurs. Birds, and their closest relations, the crocodiles, are believed to be the direct descendants of dinosaurs. The fossil evidence reveals one early bird-like ancestor, Archeopteryx lithographica, that was small and had feathers similar to modern birds, but had teeth and a bony tail similar to the dinosaurs. Below is a reconstruction of Archeopteryx (left) and a photo of the fossil itself (right) from the Smithsonian Institute.
Two reptilian features that are seen in modern birds include the amniotic egg (which allows for the embryo to develop on land without drying out) and scales on the legs. Despite being their relative, modern birds are quite different from reptiles due to the presence of feathers, their ability to thermoregulate (i.e. warm-blooded), and a specialized body designed for flight.
Basic Characteristics of Birds
Birds are bipedal (upright on two legs) vertebrates (organisms with a backbone) that are very distinctive from other vertebrates (mammals, amphibians, reptiles, and fish). The presence of feathers is a defining characteristic that distinguishes birds from other vertebrates. Feathers serve to insulate the bird, help to regulate body temperature, and aid in flight; they are dead structures that must be replaced on a regular basis. In addition, birds have bills, or beaks, that vary widely between individual species, but are always toothless and have a horny covering or sheath. Because there are no teeth to chew the seeds or insects that the bird eats, the birds' digestive system includes a gizzard that is designed to essentially "chew" the food to aid in digestion.
Birds also have large, well-developed, brains that are 6 to 11 times as large as reptiles of the same size. Bird brains, like the brains of primates, exhibit functional lateralization which means that each hemisphere of the brain is different and each has a different function. In birds the left hemisphere, which is responsible for learning and innovating vocal repertoires, is dominant. Birds use a syrinx to produce sound that is somewhat analogous to the human larynx. Birds, particularly the songbirds, have the greatest sound-producing capability of all vertebrates. Birds also have the ability to navigate by using the earth's magnetic fields, celestial cues, and there is some evidence that polarized light may also play a role in their navigation ability. In addition, they have highly-developed color vision and their hearing range is broad, encompassing sounds below the capability of the human ear.
Birds Are Designed for Flight
Birds' bodies are highly specialized, and it is these adaptations that make it possible for them to fly. One feature that significantly reduces the weight of birds is the presence of air chambers inside their bones. This serves to make the bones essentially hollow, thus allowing them to be light enough to fly. In addition, the front limbs are modified into wings rather than legs, and the overall number of bones is also reduced. Many bones have been fused into a single structure, and some bones have been eliminated entirely, which also reduces the weight of the bird. Birds have replaced jaws and teeth with a beak, and they have a slender neck that attaches the head to a rather compact torso. The sternum of birds is enlarged and has a keel, which allows for the attachment of strong flight muscles. Even the respiratory systems of birds is designed to be the most efficient possible and uses specialized lungs made of air sacs. This design not only makes respiration more efficient, by allowing air to circulate one way through the lungs so that gasses are continually exchanged, but it also lightens the bird's body to assist with flight.
Birds Vary in Size and Shape
The variety of body forms and shapes in the various species of birds across the globe is due to the fact that there are so many factors to which birds must adapt. Bill sizes and shapes change relative to the types of food that the bird eats. Terrestrial carnivores, such as hawks, falcons, eagles, and owls, have strong, hooked beaks designed to tear flesh. Other bill shapes are designed to spear fish, crack seeds, probe into flower centers to retrieve the nectar, probe cracks and crevices, or filter microscopic food from the mud.
The length of the bird's leg and the foot size and shape varies based on whether it is a perching species or one of a more terrestrial nature, and different types of locomotion have developed based on the needs of the particular species. Wading or running birds (such as storks or ostriches) have longer legs that are strong and powerful, while birds that move quickly through the air (such as Chimney Swifts) often have shorter legs and smaller feet. The feet of some birds, such as herons, are designed to help them distribute their weight evenly on soft surfaces. Climbing birds, such as woodpeckers or nuthatches, have large, curved claws that help them to grip the sides of trees.
The shape of the bird's wing also varies based on flight patterns. Some birds have longer, narrower wings that are adapted to soaring over the oceans while others have shorter, rounder wings that help them to flutter through dense undergrowth. Some birds, such as penguins, use their wings to propel them through the water.
Basic Characteristics of Birds
Birds are bipedal (upright on two legs) vertebrates (organisms with a backbone) that are very distinctive from other vertebrates (mammals, amphibians, reptiles, and fish). The presence of feathers is a defining characteristic that distinguishes birds from other vertebrates. Feathers serve to insulate the bird, help to regulate body temperature, and aid in flight; they are dead structures that must be replaced on a regular basis. In addition, birds have bills, or beaks, that vary widely between individual species, but are always toothless and have a horny covering or sheath. Because there are no teeth to chew the seeds or insects that the bird eats, the birds' digestive system includes a gizzard that is designed to essentially "chew" the food to aid in digestion.
Birds also have large, well-developed, brains that are 6 to 11 times as large as reptiles of the same size. Bird brains, like the brains of primates, exhibit functional lateralization which means that each hemisphere of the brain is different and each has a different function. In birds the left hemisphere, which is responsible for learning and innovating vocal repertoires, is dominant. Birds use a syrinx to produce sound that is somewhat analogous to the human larynx. Birds, particularly the songbirds, have the greatest sound-producing capability of all vertebrates. Birds also have the ability to navigate by using the earth's magnetic fields, celestial cues, and there is some evidence that polarized light may also play a role in their navigation ability. In addition, they have highly-developed color vision and their hearing range is broad, encompassing sounds below the capability of the human ear.
Birds Are Designed for Flight
Birds' bodies are highly specialized, and it is these adaptations that make it possible for them to fly. One feature that significantly reduces the weight of birds is the presence of air chambers inside their bones. This serves to make the bones essentially hollow, thus allowing them to be light enough to fly. In addition, the front limbs are modified into wings rather than legs, and the overall number of bones is also reduced. Many bones have been fused into a single structure, and some bones have been eliminated entirely, which also reduces the weight of the bird. Birds have replaced jaws and teeth with a beak, and they have a slender neck that attaches the head to a rather compact torso. The sternum of birds is enlarged and has a keel, which allows for the attachment of strong flight muscles. Even the respiratory systems of birds is designed to be the most efficient possible and uses specialized lungs made of air sacs. This design not only makes respiration more efficient, by allowing air to circulate one way through the lungs so that gasses are continually exchanged, but it also lightens the bird's body to assist with flight.
Birds Vary in Size and Shape
The variety of body forms and shapes in the various species of birds across the globe is due to the fact that there are so many factors to which birds must adapt. Bill sizes and shapes change relative to the types of food that the bird eats. Terrestrial carnivores, such as hawks, falcons, eagles, and owls, have strong, hooked beaks designed to tear flesh. Other bill shapes are designed to spear fish, crack seeds, probe into flower centers to retrieve the nectar, probe cracks and crevices, or filter microscopic food from the mud.
The length of the bird's leg and the foot size and shape varies based on whether it is a perching species or one of a more terrestrial nature, and different types of locomotion have developed based on the needs of the particular species. Wading or running birds (such as storks or ostriches) have longer legs that are strong and powerful, while birds that move quickly through the air (such as Chimney Swifts) often have shorter legs and smaller feet. The feet of some birds, such as herons, are designed to help them distribute their weight evenly on soft surfaces. Climbing birds, such as woodpeckers or nuthatches, have large, curved claws that help them to grip the sides of trees.
The shape of the bird's wing also varies based on flight patterns. Some birds have longer, narrower wings that are adapted to soaring over the oceans while others have shorter, rounder wings that help them to flutter through dense undergrowth. Some birds, such as penguins, use their wings to propel them through the water.
Click here for information on avian taxonomy.
Click here to access bird species profiles.
