It is an acknowledge opinion that the bird feathers arose evolutionary from horny scales of their reptile ancesotrs, primarily as a thermal insulation coating. Thanks to full development of the cardiac septum and separation of the arterial and vein blood flows, as well as due to overall increase in the level of metabolism, the birds appeared to be able to maintain constant worm body temperature, and the feathers contributed to it preservation. Subsequent modification of feathers and their adaptation to the flight is a secondary phenomenon, and the flight is secondary function, provided, however, a deep imprint on all organ systems in the birds. Feathers have a complex structure, in accordance to their the morphology and function they are divided into several groups - contour, flight, down-like, filamentous, bristles, powder down feather, etc. With a few exceptions, the avian body is covered with feathers unevenly, as they grow only in certain areas called pteriliyas, alternating with naked aptera. Worn feathers are renewed regularly during moult.
Like reptiles, the birds possess dry skin devoid of specific glands, with the exception of oil glands that produce the fatty grease plumage. The avian feet, like reptilian, are covered with scales (called podoteka), while the beak is covered with a horny cover (called rhamphotheca).
Intensive blood-oxygen enrichment is carried out both on the inhale and on exhale through the complex system of subcutaneous air sacs connected to a simply constructed lungs. The bags reduce the weight of the avian body and provide extra air tanks for breathing. The intensity of the vital processes in birds, including digestion, the work of the excretory, nervous and other systems is very high, which is associated with constant compensation of high energy cost of the flapping flight. Avian bones are pneumatized, but durable, many elements of the skeleton of the bird trunk are inosculate, which facilitate the flying. Inflexibility of the avian body is compensated by the flexibility and mobility of the neck. High sternal keel serves as a place for attachment of powerful muscles of flying. To lighten the avian skull, jaw were modified into a toothless beak.
In the flapping flight, avian wing is moved along a complex trajectory to follow aerodynamic laws most effectively. The wing is dense and dull on its leading edge, tapered and elastic to its back, with slightly concave bottom – all these features allow the air jets to flow around the wing freely and without turbulence and even to support it because of the greater density of air at the wing bottom.
Birds are egg-laying vertebrates, their breeding system have not undergone significant changes compared with reptiles. However, unlike most of the latter, all birds are characterized by nurture, i.e. by care for their offspring. Eggs are incubated by parents’ body heat during hatching, instead of exposing them to sun rays or to any other external heat sources. Eaglets, as opposed to juvenile reptiles, are in need of parental care, to more or less degrees, which consists of heating, feeding, and gradual inurement of them to self-feeding and self-protection from any danger.
Nurture, complex behavior (including mating and tournament rituals), preservation of parental pairs not only for a mating season, social life in many birds (including colonial nesting, cooperation between members of families, flocks, and multispecies associations) - all this indicates higher level of organization of nervous system in the birds than in reptiles. Of the sense organs, best developed are eyesight and hearing. Vocalization plays very important role in the avian life, and the birds exceed all other animals in richness and complexity of sound communications.
The birds had originated from the reptiles of the subclass Archosauria (thecodont dinosaurs) as late as middle of the Mesozoic. There are several alternative hypotheses concerning acquisition of ability to fly by direct avian ancestors.
Contemporary birds are the most numerous, widespread and prosperous among terrestrial vertebrates in both the number of individuals (more than 100 billion) and of taxonomic units of different rank (28-30 orders, about 200 families, 2.5-3 thousand genera, 8.5-10 thousand species). There is a lot of various ecological groups of birds adapted to various environments; however, subterranean and aquatic environments are inhabited by birds not so profoundly than by mammals.Phylogenetic relationships among bird orders are treated controversially. The current exhibition follows basically the system has been suggested by Dr. Alexander Wetmore in the middle of the 20th century. The entire display begins with penguins and ends with passerine birds.