The birds (scientific Latin: Birds) constitute a class of vertebrate animals, tetrapods, endothermic, oviparous, characterized mainly by having feathers, modified anterior locomotor appendages on wings, horn beak and pneumatic bones. Approximately 9,000 bird species are recognized worldwide.
Birds conquered the land more efficiently than reptiles. The main feature that allowed this achievement was undoubtedly homeothermia, the ability to maintain relatively constant body temperature at the expense of a high metabolic rate generated by the intense combustion of energetic food in the cells.
This feature allowed the birds, along with the mammals, to invade any terrestrial environment, including permanently freezing, hitherto unoccupied by other vertebrates.
Birds vary greatly in size, from tiny hummingbirds to large species such as ostrich and rhea. Note that all birds are birds, but not all birds are birds.
Birds are included in the order Passeriformes, constituting the richest order, that is, with the largest number of species within the group of birds.
While most birds are characterized by flight, ratites cannot fly or have limited flight, a feature considered secondary, that is, acquired by "new" species from ancestors who could fly.
Many other species, particularly island species, have also lost this ability. Non-flying species include the penguin, ostrich, kiwi, and the extinct dodo. Non-flying birds are especially vulnerable to extinction due to direct anthropogenic (habitat destruction and fragmentation, pollution etc.) or indirect (introduction of exotic animals / plants, mammals in particular) action.
A feature that favors the homeothermia In birds is the existence of a heart totally divided into four cavities: two atria and two ventricles.
There is no mixing of bloods. The right half (right atrium and ventricle) works exclusively with oxygen-poor blood, directing it to the lungs for oxygenation. The left half only works with oxygen-rich blood. The muscular-walled left ventricle pumps blood to the aorta. Thus, at all times, tissues receive richly oxygenated blood, which ensures the constant maintenance of high metabolic rates. This fact, associated with the mechanisms of thermal regulation, favors survival in any type of environment. The circulation is double and complete.
The breath: lungs and air sacs
The respiratory system also contributes to the maintenance of homeothermia. Although the lungs are small, there are air sacs, membranous pulmonary branches that penetrate through some viscera and even into long bone cavities.
The constant movement of air from the lungs to the air sacs and from the lungs allows a renewed supply of oxygen to the tissues, which contributes to the maintenance of high metabolic rates.
The skin of birds is dry, unglanded and rich in keratin which, in some parts of the body, is organized in the form of plaque, claws, horn beak and is a fundamental constituent of the legs.
Birds do not have glands on their skin. However, there is one exception: uropigial gland (or uropigiana), located in the dorsal portion of the tail and whose lubricating oily secretion is spread by the bird, with its beak, in the feathers. This adaptation prevents feathers from soaking in waterfowl and helps to understand why birds do not get wet even if they are unprotected during a rain.
Bird exclusivity: feather-covered body.
Digestion and excretion in birds
Birds consume the most varied types of food: fruits, nectar, seeds, insects, worms, crustaceans, mollusks, fish and other small vertebrates. They have a complete digestive system consisting of the mouth, pharynx, esophagus, pappus, proventriculum, gizzard, intestine, cloaca and attached organs (liver and pancreas).
When swallowed the food goes through the pharynx, fur esophagus and go to the chat, whose function is to store and soften the food. Then they go to the proventricle, which is the chemical stomach of the birds, where they suffer the action of digestive juices and begin to be digested. They then move to the gizzard (mechanical stomach) which has thick, muscular walls where the food is crushed.
Finally they reach the gut, where nutritive substances are absorbed by the body. The unused remains become feces.
The birds have a unique pouch, the cloaca, where the final parts of the digestive, urinary and reproductive system open and open to the outside. Through this bag they eliminate feces and urine and also lay eggs.