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Modern Birds The other remaining Archosaurs are birds given their own class Aves by traditional taxonomists. There are approximately 8,600 species of birds ranging is size from the tiny bee hummingbird to the ostrich. Bee hummingbird

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modern birds
Modern Birds
  • The other remaining Archosaurs are birds given their own class Aves by traditional taxonomists.
  • There are approximately 8,600 species of birds ranging is size from the tiny bee hummingbird to the ostrich.

Bee hummingbird


characteristics of the birds
Characteristics of the birds
  • Feathers a unique character among living animals, but also found in dinosaurs.
  • Endothermic
  • Skeleton modified for flight. Bones hollow, forelimbs support the wing, ribs with uncinate processes, beak but no teeth, reduced tail.
  • Breathing by lungs and associated air sacs
  • Internal fertilization and hard-shelled amniotic egg
evolution of birds
Evolution of birds
  • Birds evolved from a group of theropod dinosaurs in the Jurassic period. The oldest known bird fossil is Archaeopteryx lithographica which has a mix of “reptilian” and avian features.
  • Reptilian: long tail, teeth, long clawed fingers
  • Avian: feathers, ribs with uncinate processes, avian shoulder girdle.


(oldest known

fossil bird)



  • Among living animals feathers are a uniquely avian trait.
  • However, it is now well established that feathers also occurred in dinosaurs. In the 1990’s feathers were described from series of non-avian coelurosaurs, mostly from the Chinese Liaoning deposits.
  • Feathers are what enable birds to fly, but originally are believed to have evolved as a thermoregulatory device.
  • Feathers are lightweight, but strong. The surface of the feather is made up of tightly spaced, overlapping filaments that hook together. Overlapping feathers form the wings with which birds fly.
feather structure
Feather structure
  • Feathers are made of keratin: an inert substance that consists of insoluble microscopic filaments embedded in a protein matrix.
  • Keratin is the substance found in hair, nails, claws and scales of other animals, but bird keratin is unique and differs from that of modern reptiles.
  • There are two main categories of feathers
    • Plumaceous – downy for insulation
    • Pennaceous – linked , vaned feathers wing and contour feathers.
  • Vane of a typical body feather consists of a hidden downy base (for insulation) and an exposed cohesive outer portion (for streamlining).

Downy Feather

  • Body feathers of most birds include an aftershaft that emerges from the underside of the shaft where the first basal barbs of the vane branch off.
  • The aftershaft is almost always downy and functions to increase insulation. In ptarmigan winter plumage the aftershaft is ¾ as long as the main feather.



feather structure15
Feather structure
  • A contour feather has a long central shaft and a broad flat vane. The hollow base of the shaft (quill) anchors the feather in a follicle under the surface of the skin.
  • The rest of the shaft, the rachis, supports the vanes. Branching off from the rachis are barbs. Each barb has barbules projecting to either side that interlock with the barbules of adjacent barbs.
  • Barbs and barbules form an interlocking, but flexible surface.
adaptations for flight
Adaptations for flight
  • In general, the avian skeleton has been lightened and strengthened for flight.
  • This has been achieved by eliminating some structures and modifying others.
adaptations for flight18
Adaptations for flight
  • Feathered wing.
  • Mass reduction
    • Wrist bones reduced to two
    • Bones hollow and supported by internal struts or spongy bone
    • Reptilian tail lost. Fused tail bones (pygostyle) support tail feathers
    • Teeth lost. Skull and bill light but strong.
adaptations for flight20
Adaptations for flight
  • Skeleton strengthened
    • ribs have rear-facing uncinate processes that overlap and strengthen walls of thorax
    • bones of wrist, pelvis fused
  • Sternum or breastbone enlarged with a large keel (carina) for attachment of massive flight muscles -- pectoralis and supracoracoideus.
  • Fused hand bones support and maneuver primary flight feathers.
  • Efficient lungs and powerful four-chambered heart power flight.
further skeletal modifications for flight triosseal canal
Further skeletal modifications for flight: Triosseal canal
  • Trisossealcanal. The triosseal canal is formed by the junction of the coracoid, scapula and furcula. The supracoracoideus tendon passes through this canal up from the supracoracoideus and inserts on the dorsal face of the humerus.
  • Dorsal insertion allows triosseal canal to act as a pulley and the supracoracoideus can lift the wing during the recovery stroke.

Triosseal Canal.

further skeletal modifications for flight triosseal canal24
Further skeletal modifications for flight: Triosseal canal
  • If the supracoracoideus tendon is cut a bird cannot takeoff because the supracoracoideus muscles are required for the initial rapid wingbeats necessary to get a bird off the ground.
  • Once airborne a bird with severed supracoracoideus tendons can fly because the dorsal elevator muscles are capable of raising the wing on the recovery stroke.
further skeletal modifications for flight
Further skeletal modifications for flight
  • Furcula The clavicles are fused to form a structure called the furcula or wishbone.
  • The furcula flexs during flight and spreads and contracts during each wingbeat. The flexing may enhance gas exchange by assisting in moving air through the air sacs.

Furcula (in red)

further skeletal modifications for flight arm and fingers
Further skeletal modifications for flight: arm and fingers
  • The wing is supported by the arm and finger bones. There are three remaining fingers. The fused hand and finger bones provide strength and rigidity in the outer wing skeleton.
  • The primary feathers attach to the wrist and finger bones, while the secondaries attach to the radius and ulna. The first digit (the thumb if birds had one) controls the alula or bastard wing.
  • The alula is a flap that is important in maintaining a smooth flow of air over the wing at low speeds (it helps prevent stalling).
bernoulli s principle
Bernoulli’s Principle
  • A bird’s wing is an airfoil and is cambered with a slightly convex upper surface and concave under surface.
  • Because air must travel further over the upper surface of the wing than below it must travel faster and thus exerts less pressure above the wing than it does below.
  • The increased pressure below generates lift, the force which keeps the bird up.
lift and thrust
Lift and thrust
  • In order to fly both horizontal thrust and vertical lift are required.
  • Thrust is mainly generated by the primary feathers (the long ones at the end of the bird’s hand), which on the downstroke twist and acting like a propeller push the air backwards.
  • Lift is mainly generated by the secondary feathers (the inner portion of the wing), which form an airfoil.

Arrangement of feathers

structure of hindlimbs
Structure of hindlimbs
  • Legs joints and bones. When looking at a birds leg what appears to be the knee is not. It is the ankle.
  • The knee joint is hidden by feathers. The long bone leading from the toes is the tarsometatarsus (from tarsals and metatarsals) and above that is the tibiotarsus

  • Being able to perch in trees was an early avian adaptation.
  • The largest group of birds the Passerines (Passeriformes: perching birds) have four moderately long toes, three facing forward and one back.
  • The tendons of the toes can lock the foot in a firm grip. Hence a sleeping bird does not fall off its perch.

Perching Song Thrush


hopping and walking
Hopping and walking
  • Most passerines hop and in fact most cannot walk. However, some species that spend a lot of time on the ground such as larks and starlings can walk.
  • In many other groups of ground-dwelling birds (e.g. chickens and their relatives) however walking is used.
  • A few birds are specialized for running and possess long legs.
  • The most obvious are the ostrich and relatives. As is the case in horses the number of toes has been reduced to reduce mass at the end of the limb. Ostriches have two toes and rheas three.

Ostrich foot:


  • Various birds including woodpeckers, nuthatches, treecreepers and woodcreepers climb up and down tree trunks.
  • The feet in all cases are strong and the toes usually well curved and the tail is often used as a brace to prop the body against the trunk.

Montane Woodcreeper


Bennett’s Woodpecker


  • A wide variety of birds swim and webbing of the front three toes has evolved independently at least four times and all four toes are connected by webbing in the pelicans and cormorants.
  • The other adaptation to increase surface area for swimming is lobed toes.

Pelican Feet


Coot Feet

avian lung
Avian lung
  • The metabolic demands of flight are high and to meet these demands the avian lung must be much more efficient than the mammalian lung.
  • In birds the finest branches of the bronchi do not end is saclike alveoli. Instead they form tubelike parabronchi through which air flows in only one direction (in contrast to the tidal flow in mammals). Because the air flow is one-way, gases can be more efficiently exchanged and there is no “dead air” in the lungs
avian lungs
Avian lungs
  • The one-way flow of air is achieved by using a system of air sacs and a two breath cycle.
  • On inspiration a bolus of air flows down the trachea to air sacs below the lung. On expiration the air mass flows into the lung where gas exchange takes place.
  • With a second inspiration the air mass in the lung flows into anterior airsacs and with a second expiration exits the body via the trachea.
feeding and digestion beaks
Feeding and digestion: beaks
  • Birds are not the only organisms with beaks as turtles and various dinosaurs possessed them too.
  • However, birds have a tremendous diversity of beaks.
  • The beak is a birds principal tool for handling food and its size and shape determine the foods that can be eaten.
  • Birds such as crows and ravens have a general purpose bill that can handle lots of different foods.
  • Warblers have fine forceps like bills good for handling soft-bodied foods such as insects.
  • Finches possess short, deep bills that are ideal for opening seeds. Crossbills have beaks in which the upper and lower mandibles cross.
  • These are ideal for opening pine cones to reach the seeds




Magnolia Warbler


House Finch


Crossbill beak


  • Dabbling ducks, such as mallards, have beaks that are good for straining water. However, ducks such as mergansers have beaks that are serrated and help in grasping fish.
  • Spoonbills and flamingoes possess bills similar to dabbling ducks that are good for sifting mud and water.

Common Merganser:


Merganser beak


Flamingo beak


  • Raptors, such as owls, hawks and eagles, possess short bills with a hooked tip that are ideal for dismembering prey.

Ferrunginous Hawk

  • Toucans and hornbills are both fruit specialists and their light, but long bills allow them to pluck distant fruit.

Keel-billed Toucan

Great Hornbill:

  • Among the more unusual beaks is the beak of the skimmer a relative of the gulls and terns.
  • In skimmers the lower mandible is laterally compressed and the bird flies over the water with the lower mandible skimming through the water. When the beak hits a fish, it snaps shut immediately on it.

Black Skimmer

feeding and digestion
Feeding and digestion
  • Because birds lack teeth they can’t process food much in the mouth so that is left up to the gastric system.
  • Birds can frequently gather food faster than it can be processed. This food is usually stored in the birds crop, an enlarged part of the esophagus.
  • The crop is also used to store food that will later be regurgitated to chicks.

Bird’s digestive tract and other internal organs

  • One South American bird, the Hoatzin is an obligate leaf eater and this is the only bird that uses its crop as a fermentation chamber.
  • Hoatzins are also unusual because their chicks possess claws on their wings, which they use to scramble around in the branches. The claws are later lost.

Adult Hoatzin


  • The stomach a birds possesses is dictated by its diet. Birds that each high volumes of soft foods such as meat have extensible stomachs that can hold a lot of food.
  • Those that have to process tough foods such as seeds and insects have much more muscular stomachs that help to grind the food.
  • A bird’s stomach has two parts the anterior glandular proventriculus and the posterior gizzard.
  • The proventriculus contains glands that secrete digestive enzymes. In birds that swallow whole foods such as fruits the proventriculus is often very large.
  • The gizzard’s main function is to mechanically process food.
  • The walls of the gizzard are thick and muscular and the gizzard often contains small stones, which the birds swallow to assist in grinding the food. The gizzard thus fulfills the same role as the teeth in mammals.
  • The gizzard can exert significant pressure. For example, a turkey’s gizzard can process two dozen walnuts in about four hours.
  • It can also crack hickory nuts, which require 50-150kg of pressure to break.
  • The main site of chemical digestion is the intestine where enzymes break down the food into small molecules that can be absorbed across the intestinal wall.
seasonal changes in gut morphology
Seasonal changes in gut morphology
  • Birds often change their diets over the course of a year and gut morphology changes too. Insects are more easily and quickly digested than plant food (e.g. berries).
  • When starlings switch to eating more plant material in the fall their intestines increase in length by about 20% and decrease by a similar amount in spring when their diet switches back to animal prey.
  • Accompanying the morphological changes are changes in the types and quantities of digestive enzymes produced tailored to match the composition of the diet.
sensory systems vision
Sensory systems: vision
  • Most birds have excellent vision and this is reflected in the structure of the brain. There are large optic lobes and the midbrain which processes visual information is enlarged.
  • In contrast, in most birds olfaction is unimportant and the olfactory bulbs are small.

Puerto Rican Screech owl


  • Birds have very large eyes so much so that the brain is up and back in the skull to accommodate them.
  • Bird eyes are similar in structure to those of other vertebrates, but the shape varies from a flattened sphere to tube-like.
  • The variation is shape appears to be a result of the difficulties of fitting an enlarged eye into a more modest sized skull. By altering eye shape birds such as owls have avoided developing the disproportionately large heads they would have required if the eye were spherical.
A unique feature of the avian eye is the presence of a comb-like structure called the pecten.
  • The pecten arises from the rear of the eye close to where the optic nerve exits the eye.
  • The function of the pecten remains unclear even after 200 years of investigation. The organ’s large blood supply suggests it may provide nutrition to the retina and perhaps remove metabolic wastes from the vitreous humor.
  • A second interesting feature of avian eyes is the presence of colored oil droplets in cone cells.
  • These act as filters absorbing certain wavelengths of light and allowing others through, but their exact function remains unclear.
  • Birds have hearing that is comparable in sensitivity to that of humans even though their heads are much smaller.
  • However, they have proportionally much larger tympanic membranes which enhances sensitivity to sound. In addition, the cochlea has about 10x as many hair cells per unit length than a mammalian cochlea does.
  • Owls possess the most acute hearing among birds (comparable to that of a cat) and can isolate sounds very accurately even in complete darkness.
  • Owls possess a distinctive facial ruff of stiff feathers that acts as a parabolic sound reflector, which focuses and amplifies sounds.
  • Some ruffs are asymmetric and the ruff’s asymmetry (as well as asymmetry in the vertical placement of the ears) enhances the owls ability to isolate sounds in three dimensional space.

Barn owl


  • The asymmetries in the ruff and ears cause delays in the time at which sounds reach each ear that can be interpreted by the brain and used to identify precisely the source of a sound.
  • A barn owl’s ability to do this is so good that it can isolate sound to within 1º in three dimensional space. If you envisioned yourself surrounded by a sphere with a radius approximately equal to your arm length 1º would be about the area covered by a fingertip.
  • Most birds have a poorly developed sense of smell, but a few groups do have a good sense of smell.
  • These include kiwis which have their nostrils at the end of the bill and use odor cues to find prey when they probe in the earth.

Brown Kiwi hatched at the Smithsonina National Zoo 2006:

  • Other birds with a good sense of smell are the various “tubenoses” the petrels, shearwaters and albatrosses, which are attracted to the scent of chummed fish and fish from a considerable distance.
  • In addition, turkey vultures have a well developed sense of smell and can find even covered carcasses very quickly.


Turkey Vulture:


diversity of birds major bird orders
Diversity of birds: major bird orders
  • Struthioniformes: Ostriches, rheas, cassowaries
  • Podicipediformes – Grebes (diving birds with lobed feet). 21 species.
  • Gaviiformes – Loons. 5 species.
  • Pelecaniformes. Pelicans and cormorants. 67 species.

Southern Cassowary


Pied-billed Grebe


Common Loon


Great Cormorant

diversity of birds major bird orders93
Diversity of birds: major bird orders
  • Anseriformes – Waterfowl. Ducks, geese and swans. About 160 species.
  • Falconiformes Hawks, eagles and falcons. Over 300 species
  • Galliformes – chicken-like birds. Grouse, quail, turkeys. Over 250 species.

Ruffed Grouse

diversity of birds major bird orders95
Diversity of birds: major bird orders
  • Ciconiiformes – Long-legged wading birds. Storks, herons, ibises, egrets. 120 species.
  • Charadriiformes – Shorebirds and gulls. Plovers, sandpipers, stilts, auk, puffins, gulls and terns. Over 360 species.
  • Columbiformes – Pigeons and doves. Over 300 species.
  • Psittaciformes Parrots. More than 350 species

Razorbill (auk)


White Stork



Rock Dove,Feral.jpg


Scarlet Macaw


diversity of birds major bird orders99
Diversity of birds: major bird orders
  • Strigiformes – Owls. About 180 species.
  • Apodiformes – Short-legged acrobatic fliers. Swifts and hummingbirds. About 420 species.
  • Piciformes – Woodpeckers. About 400 species.

Common Swift

diversity of birds major bird orders101
Diversity of birds: major bird orders
  • Passeriformes – Perching birds. Songbirds. Crows, warblers, cardinals, sparrows, starlings, etc. Over 5,700 species.




European Starling