Monday, May 08, 2006


Birds are warm blooded, bipedal, egg laying vertebrate animals with feathers an fore limbs modified to wings, and have hollow bones1. Birds are closely related to reptiles as compared to other groups of animals. Birds evolution began in the early Triassic (225-200 million years ago) by small pseudosuchians like the Saltoposuchus being able to walk on two feet (bipedal) 4. By being bipedal the birds freed their fore limbs. The modern tree perching bird evolved 135 million years ago as shown by a fossil found in China. They are believed to evolve from the Maniraptora group Theropod dinosaurs1, 4.

The bird’s skeleton is well adapted for flight. First the bones of the birds are hollow making them light but strong enough to withstand takeoff, flight and landing stresses. The numbers of hollow bones vary from bird to bird with the bigger birds having more. The bird’s bones are reinforced by other bones to make the structural formation strong3. Flightless birds like the penguins like the have solid bones Birds bones are fused reducing the number of bones birds have, actually bird have fewer bones than reptiles and mammals3. Birds have more neck bones (cervical vertebrae) making its neck long and flexible than many other animals and are the only animals with a fused collarbone.3This features make birds lighter and well adapted for flight.

Birds require a lot of oxygen especially when they are on flight due to their high metabolism1. And birds have a special mechanism for gaseous exchange where air inhaled and exhaled doesn’t mix in the lungs as in the case of mammals1. This happens because other than having lungs birds also have posterior and anterior air sacs1. The lungs and the sacs will expand during inhalation and air enters through the trachea, half goes into the posterior sacs and half of the air goes into the lungs and then to the anterior sacs. During exhalation the lungs, anterior and posterior sacs contract to expel air1. The posterior sac releases air directly into the trachea and anterior releases air in to the lungs and the lungs to the trachea and out. This way the air from the body which is rich in carbon dioxide doesn’t mix with the one being taken in and is rich in oxygen. This provides birds with clean air for metabolism while still on the move1.

Birds don’t have to carry their eggs after fertilization which becomes more of an advantage for the birds during flight as they wouldn’t be very heavy as in case they produced by vivipary.

On top of having light bones birds have feathers. Feathers evolved to insulate birds from cold and are thought to have evolved from the scales and arising from papillae2, 4. Flight evolved as a secondary function. Papilla grows from epidermal pit called the feather follicle. A feather is made of a stiff shaft which attaches to the body at a point which is called the quill4. The shaft has two rows of attached branches called barbs which have other smaller branches called barbules4. The whole formation of barbs and barbules is called the vane which is light but very resilient2, 4. This light weight is vital for flight as the bird requires carrying little weight. Each feather is shed at least once a year being replaced by a new one. Birds constantly repair their feathers using their beaks and also applying special oil found at the tail to sort of water proof them4.

Depending on the size and the lifestyle of the birds’; different birds have different wing formations. Small birds have wings that help them to fly fast to escape from enemies, whereas there bigger and big birds preferring to fly high and also utilise warm air currents high up4. The shape of the bird is in such a way that it offers little resistance to the air. They also have low centres of gravity by having wings attached high on the trunk and having light organs like the lungs and having digestive muscles placed ventrally4. The flight muscles are also placed ventrally allowing the bird to be light and be overturned by the gravitational pull4. The shape of the wings determines the speed and how long the bird can fly. High flying birds have pointed wings which enable them to soar high for long durations using less energy. An example is the swifts and swallows. Sparrows have short rounded wings that enable them to take off fast and rapid flight over short distances4. Vultures have low speed cyclic flights to utilise thermal air currents high up and so they have rectangular wings4. Humming birds are able to achieve a hovering flight by tilting their bodies to almost upright position and beating their wings rapidly4.


1. Wikipedia contributors. Bird [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 8, 02:43 UTC [cited 2006 May 8]. Available from:

2. Wikipedia contributors. Feather [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 7, 20:46 UTC [cited 2006 May 8]. Available from:

3. Wikipedia contributors. Bird skeleton [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 7, 21:03 UTC [cited 2006 May 8]. Available from:

4. Knight, R. BCB Biodiversity chapter 2 Lords of The Air (Cited 2006 May 6)


Biodiversity and Conservation Biology

University of the Western Cape

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