Pigeons are popular perching birds, which along with doves are assigned to the taxonomic family Columbidae. They are perhaps best known for their domestic use by pigeon breeders and for the service of homing pigeons as mail carriers.
Pigeons live in tropical and warm regions. These birds have short, thin necks, small heads, and heavy plumage. The pigeon is recognizable by their head-bobbing stride. This bird is found almost worldwide divided among 175 species. These species include the new and old world species and the old world stretopelia species. 
An adult pigeon has red and orange eyes. Young pigeons less than 6 to 8 months old have brown or gray eyes. Pigeons have exceptionally outstanding eyesight. Pigeons can distinguish colors like humans do, however the pigeon’s eyesight is able to detect ultraviolet light that humans can’t see. Because of that outstanding ability, pigeons are often used in human rescue missions. Pigeons have not only exceptional eye sight but also an extensive system of interconnected air sacs. These bring fresh air to the lung even when the pigeon is breathing out. As much as 75% of the air bypasses the lung and is stored in the air sacs. During expiration, this stored air is available for its lungs so that they can get fresh air during both inspiration and expiration.
The pigeon reproduces throughout the year, and can raise about five broods every year, even during early winter. The male bird keeps the nest during the day; the female bird at night. The female bird lays two or three white eggs. Both parents usually take turns keeping eggs warm. Eggs hatch after 16 to 19 days and the young birds feed on crop milk for the first two weeks. Finally seeds take the place of the crop milk. 
Pigeons' problems and distribution
Pigeons, in spite of their beauty and friendliness for humans, have caused several problems by their droppings and by creating aesthetic problems. Their droppings can damage man-made creations such as bronze statues, buildings, and bridges, because of the acidic condition of their droppings. 
Pigeons are distributed world-wide in Eurasia, Africa, and America. There are somewhat differentiated species of pigeons that have different names in different areas. For instance, the most popular North American pigeon is the mourning pigeon, and the smallest pigeons which can be mainly found in southern US and in the New World tropics are called sparrow-sized ground pigeons.
The pigeon’s general habitat is rocky cliffs, and they have adapted to buildings as if they were cliffs. Pigeon nests are often located on concealed building ledges that look like high cliffs. The pigeon’s nest is usually made with branches (sticks) and is well hidden. Usually the male bird brings the nesting substance to his mate, and when the time comes, the female bird constructs the nest. After 2-3 days, the pigeon nest is usually complete. 
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