Birds
Birds (class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species of birds, making them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2-inch) Bee Hummingbird to the 2.75 m (9 feet) Ostrich. Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings; the now-extinct flightless Moa of New Zealand was the only exception.
Figure 14. Five ducks swimming on a pond.
"Ducks Swimming" by Michael Ayers, C3BC is licensed under CC BY 3.0
Bird Skeletons
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) that connect with the respiratory system. The skull bones in adults are fused and do not show cranial sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar, and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The latter few are fused with the pelvis to form the synsacrum. Parts of the vertebral skeleton and braincase are fused to increase its strength while lightening its weight. Most species of bird only possess one ovary rather than two, and no living birds have teeth in their jaw, further reducing body mass. The ribs are flattened and the sternum is keeled for the attachment of large flight muscles except in the flightless bird orders. The forelimbs are modified into wings.
Cardio-Respiratory System
Birds have one of the most complex respiratory systems of all animal groups, as flight requires enormous energy expenditure. Upon inhalation, 75 percent of fresh air bypasses the lungs and flows directly into a posterior air sac that extends from the lungs and connects with air spaces in the bones, filling them with air. The other 25 percent of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx-- a muscular chamber incorporating multiple tympanic membranes-- that diverges from the lower end of the trachea.
The bird's heart has four chambers like a mammalian heart. In birds the main arteries taking blood away from the heart originate from the right aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the aorta. The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating red blood cells in birds retain their nucleus.
Nervous System
The nervous system is large relative to the bird's size. The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behavior patterns, navigation, mating, and nest building. The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Birds are tetrachromatic, possessing UV-sensitive cone cells in the eye as well as green, red and blue ones. This allows them to perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Ultraviolet light is also used in foraging —kestrels have been shown to search for prey by detecting the UV-reflective urine trail marks left on the ground by rodents. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field.
Reproduction
Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: Plainer males choose brightly colored females. Courtship feeding, billing, and allopreening are commonly performed between partners, generally after the birds have paired and mated.
Male and female birds have a cloaca, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, which is sometimes known as the "cloacal kiss," during which the male transfers his sperm to the female. A few species of birds (e.g. most waterfowl) have an intromittent organ that is known as a phallus and is analogous to the mammal's penis. The female lays amniotic eggs in which the young gestate. Unlike most vertebrates, female birds typically have only one functional ovary and oviduct, and gonads in both species are small except during breeding season--weight-saving adaptations that are useful for flight. They also have no urinary bladder. All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. As a group, birds, like mammals, are noted for their high level of parental care.
Figure 15. Yellow bird perched on a flower.
"Yellow Bird" by Michael Ayers, C3BC is licensed under CC BY 3.0