Birds

Merriam-Webster dictionary defines birds as warm-blooded vertebrates that have feathers and wings in the Aves class.http://www.merriam-webster.com/dictionary/bird Their vision is sharp.http://www.britannica.com/EBchecked/topic/66391/bird However, they have an inferior sense of smell.http://www.britannica.com/EBchecked/topic/66391/bird They also have light bones that assist in flight. Birds incubate their offspring in eggs (with the exception of a few species).http://www.britannica.com/EBchecked/topic/66391/bird Some birds can fly while others can not.http://www.britannica.com/EBchecked/topic/66391/bird The penguin is an example of a flightless bird.

It is believed that birds evolved from dinosaurs.http://www.birds.com/education/evolution/ Some scientists trace the evolution of birds back to the Archaeopteryx lithographica fossil discovery, which they believed evolved from the theropod dinosaur.http://www.birds.com/education/evolution/ However, there are a minority of scientists who disagree with this theory.http://www.birds.com/education/evolution/ They believe that birds evolved from four-legged reptiles.http://www.birds.com/education/evolution/

Scientists also do not agree on why birds developed feathers. Some believe it was to provide insulation while others think they came about because of the need for flight.http://www.birds.com/education/evolution/ They theorize that flight gave birds the ability to beat out rival animals for food and to survive longer because they could live beyond the reach of predators.http://www.birds.com/education/evolution/

According to the classification system developed by Carl Linnaeus, birds are listed in the Kingdom Animalia, Phylum Chordata and Class Aves.http://www.birds.com/species/classifications/ There are 23 orders, 142 families and 9,702 species.http://www.birds.com/species/classifications/ Of this number, 42 species and 44 subspecies have become extinct.http://www.birds.com/education/extinct-rare/

Bird feathers are made up of layers. The shape of the feather is controlled by the contour feathers while the inside down feathers keep the bird warm.http://www.birds.com/education/anatomy/ Feathers are made from keratin.http://www.birds.com/education/anatomy/ They cling together to form a seal that helps manage wind resistance.http://www.birds.com/education/anatomy/ Because the feathers weigh more than the bird's bones, it is able to fly.http://www.birds.com/education/anatomy/ The bird bone has a unique honeycomb structure that keeps its weight light but still maintains its strength.http://www.birds.com/education/anatomy/

Birds (that can fly) have a keel that holds the flight muscles.http://www.birds.com/education/anatomy/ These muscles come in a red and white form. Red flight muscles are used for long distance flying.http://www.birds.com/education/anatomy/ White flight muscles are used for speedy take offs.http://www.birds.com/education/anatomy/