Describe Class Aves in details?

Class Aves: The fossil record contains evidence that birds evolved from the reptiles. Birds share anatomical features with Protoavis and Archaeopteryx, both flying reptiles that date back 225 million and 150 million years, respectively. Birds do have one major feature that distinguishes them from the reptiles and all other animals-feathers. However, feathers are thought to represent modified reptilian scales.

Feathers, along with other structural modifications, enable birds to fly. Feathers are built from hollow rod-like structures that provide for a high strength-to-weight ratio. They are made of the same biomolecule that makes hair and fingernails-the protein keratin. Each feather consists of a main hollow shaft called the rachis. The rachis branches into barbs, and the barbs themselves branch into barbules. Upon close magnification, the barbules of the feathers can be seen to have hooks that form interlocking units with other barbules, making the feather sturdy and able to retain its flexibility and shape.

You may have noticed birds going through a curious behavior called preening. When a bird preens, it takes each feather and draws it through its beak. Birds use their beaks to take oil secreted from a preen gland at the base of their tail to condition the feather, and to align the barbules and interlocking hooks.

Feathers have two major functions. One is to form airfoils (wings that provide lift) for flight: feathers that do this are flight, or contour feathers. The other function is to provide insulation against heat loss. Soft, down feathers lack the interlocking barbules, and instead allow for random, fluffy arrangements of the barbs that trap insulating air in pockets.

Several other adaptations enable birds to fly. They have very thin, light, yet strong skeletons. In fact, many of their bones are hollow; some have structural crisscrossed braces, which add strength. Many bones are fused, adding strength as well. Along with the skeleton, birds possess powerful breast muscles to move their wings.

Birds are warm-blooded. Being homeothermic, or endothermic, they are able to regulate their body temperature internally. In order to maintain their temperature range, they must eat voraciously and/or increase their rate of breathing. As such, they are able to live in either warm or cold climates. To reiterate, their feathers also provide a large measure of insulation to maintain stable temperature environments.

Form follows function, in that a bird's beak and feet provide clues as to what it feeds on and where it lives. Think about a duck's beak and its webfeet, or predators like hawks and eagles, with their sharp talons and beaks. Ducks have beaks that enable them to feel for food in the mud and webbed feet for paddling through water.

Birds reproduce by laying eggs, and fertilization is internal even though males do not have penises. Males and females must bring their cloacas together in order to transfer sperm. A cloaca is the common opening through which waste products as well as reproductive products pass. Much of bird behavior is related to courtship rituals and displays, which can be quite sophisticated.

Birds are known to possess keen visual acuity as well as hearing. Birds can have eyes on the sides of their heads, allowing wide fields of vision; or their eyes may face forward, as in many birds of prey, in order to have binocular vision with good depth perception. Flying also brings with it the requirement to navigate, and we know that migratory birds perform this function very well!