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By the end of this section, you will be able to:
  • Describe the evolutionary history of birds
  • Describe the derived characteristics in birds that facilitate flight

The most obvious characteristic that sets birds apart from other modern vertebrates is the presence of feathers, which are modified scales. While vertebrates like bats fly without feathers, birds rely on feathers and wings, along with other modifications of body structure and physiology, for flight.

Characteristics of birds

Birds are endothermic, and because they fly, they require large amounts of energy, necessitating a high metabolic rate. Like mammals, which are also endothermic, birds have an insulating covering that keeps heat in the body: feathers. Specialized feathers called down feathers are especially insulating, trapping air in spaces between each feather to decrease the rate of heat loss. Certain parts of a bird’s body are covered in down feathers, and the base of other feathers have a downy portion, whereas newly hatched birds are covered in down.

Feathers not only act as insulation but also allow for flight, enabling the lift and thrust necessary to become airborne. The feathers on a wing are flexible, so the collective feathers move and separate as air moves through them, reducing the drag on the wing. Flight feathers are asymmetrical, which affects airflow over them and provides some of the lifting and thrusting force required for flight ( [link] ). Two types of flight feathers are found on the wings, primary feathers and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust. Secondary feathers are located closer to the body, attach to the forearm portion of the wing and provide lift. Contour feathers are the feathers found on the body, and they help reduce drag produced by wind resistance during flight. They create a smooth, aerodynamic surface so that air moves smoothly over the bird’s body, allowing for efficient flight.

The illustration shows a bird’s wing, which has two layers of flight feathers, the long primary feathers and the shorter secondary feathers, which overlay the primary feathers.
Primary feathers are located at the wing tip and provide thrust; secondary feathers are located close to the body and provide lift.

Flapping of the entire wing occurs primarily through the actions of the chest muscles, the pectoralis and the supracoracoideus. These muscles are highly developed in birds and account for a higher percentage of body mass than in most mammals. These attach to a blade-shaped keel, like that of a boat, located on the sternum. The sternum of birds is larger than that of other vertebrates, which accommodates the large muscles required to generate enough upward force to generate lift with the flapping of the wings. Another skeletal modification found in most birds is the fusion of the two clavicles (collarbones), forming the furcula    or wishbone. The furcula is flexible enough to bend and provide support to the shoulder girdle during flapping.

An important requirement of flight is a low body weight. As body weight increases, the muscle output required for flying increases. The largest living bird is the ostrich, and while it is much smaller than the largest mammals, it is flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones are bones that are hollow, rather than filled with tissue ( [link] ). They contain air spaces that are sometimes connected to air sacs, and they have struts of bone to provide structural reinforcement. Pneumatic bones are not found in all birds, and they are more extensive in large birds than in small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are.

Questions & Answers

what is biology
PEACE Reply
what is biology
Ysabella Reply
biology is a study of living things
PEACE
Biology is a diverse branch of science that deals with mostly living things
Emmanuel
yes
Swapnil
What happen when inhibit the transcription?
Swapnil
what is the effect of not doing sexual intercourse
SUZAN Reply
what is the mechanism of cellular respiration
Rita Reply
what is enzyme
garry Reply
They are organic catalysts that alter the rate of chemical reactions in the body.
Iyadi
what is a cell
Praize Reply
what is biology
Mordi Reply
biology is the study of living organisms and their interactions with one another and their environments
Ysabella
which of the following event does not occur during some stages of interface?
Bangha Reply
What is microfilaments
KHalid Reply
What is multicellular organisms
Ovie Reply
these are organisms with more than two cells
Bangha
the process when a male toad fertilizer a female eggs is called what?
Ahrebe Reply
Fertilization
Gyamfi
how did unicellular organisms form plants and animals or is it that different unicellular organisms formed plants and animald
YXNG Reply
yes
James
thanks
YXNG
name the components of faeces
Damali
undigested carbohydrate, fibre
Sandra
what are unicellular organisms..?
Sackson
they have only one cell
Sandra
faeces contains many undigested food materials, after the food has been digested then it will be absorbed in the blood stream for assimilation.,......... but the remains toxic materials are stored in the rectum these toxic materials are the faeces and it contains bile salts, the polysaccharides .
James
nice.
Sandra
thanks
James
unicellular organisms are the ones with only single cell.
James
thanks for your answers guys.
Sackson
Ok
Richard
what is class bryophyta
Emefa Reply
how many stages do we have in glycolysis?
Damali
10 stages
Elisha
the presence of a membrane enclosed nuclosed is a characteristics of what
Addai Reply
eukaryotic cell
captain
hetreothalism in fungi
Lekhram Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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