Birds

avian dinosaurs

Phylum Chordata

Subphylum Vertebrata

– Class Agnatha– Class Chondrichthyes– Class Osteichthyes– Class Amphibia– Class Reptilia– Class Aves– Class Mammalia

Characteristics of reptiles Vertebrate

animals Lungs Scaly skin Amniotic egg

Amniotic Egg – Reptiles and Birds

Reptile eggs have a leathery shell

Has several membranes

Contains yolk rich in nutrients for embryo

Amniotic Egg Membranes Amnion – thin

membrane holds the fluid the embryo floats in

Yolk sac – holds the yolk (fatty food) for the developing embryo

Allantois – holds embryos nitrogenous wastes

Chorion – surrounds all the other membranes; protects embryo

Pterosaurs = hang glider wings.

Not a bird ancestor

The Origin of Birds Birds descended from theropods – a group of small,

carnivorous dinosaurs. By 147 million years ago, feathered theropods had

evolved into birds.

The Origin of Birds

Archaeopteryx– The oldest bird known.– Skull similar to modern birds but with thecodont

teeth.– Wings with feathers were present.

The Origin of Birds

Much of the skeleton was that of a theropod dinosaur.– Long bony tail

– Clawed fingers

– Abdominal ribs

– S-shaped, mobile neck This fossil demonstrated

the connection between theropods & birds.

Archaeopteryx(oldest knownfossil bird)Jurassic 150mya

• mostly terrestrial• reptile features – eggs, scales, skull (Archaeopteryx)

Class Aves - birds

The Origin The Origin of Birdsof Birds

Archaeopteryx arose from the theropod lineage.

Closely related to Dromaeosaurs.– More shared

derived characters.

– Many may have used feathers for insulation and/or social display..

Modern Birds

The two surviving Archosaurs are crocodiles and 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

ostrich

Adult Hoatzin

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

FeathersFeathers

Among living animals feathers are a Among living animals feathers are a uniquely avian trait.uniquely avian trait.

However, it is now well established that However, it is now well established that feathers also occurred in dinosaurs. In feathers also occurred in dinosaurs. In the 1990’s feathers were described from the 1990’s feathers were described from series of non-avian coelurosaurs, mostly series of non-avian coelurosaurs, mostly from the Chinese Liaoning depositsfrom the Chinese Liaoning deposits..

Feathers

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.

Dinosaur feather impressions

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.

Feathers

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

Form & Function – FeathersForm & Function – Feathers

Feathers are homologous to reptiles’ scales.Feathers are homologous to reptiles’ scales. It develops from an epidermal elevation It develops from an epidermal elevation

overlying a nourishing dermal core.overlying a nourishing dermal core.– In reptiles, this elevation flattens into a scale.In reptiles, this elevation flattens into a scale.– In birds, it rolls into a cylinder and sinks into In birds, it rolls into a cylinder and sinks into

the follicle from which it will grow.the follicle from which it will grow.

Form & Function – FeathersForm & Function – Feathers

As a feather nears the As a feather nears the end of its growth, end of its growth, keratin is deposited to keratin is deposited to make some of the make some of the structures hard.structures hard.

The protective sheath The protective sheath surrounding the new surrounding the new feather splits open, and feather splits open, and the feather unfurls.the feather unfurls.

Form & Function – FeathersForm & Function – Feathers

Contour feathersContour feathers are vaned are vaned feathers that cover and feathers that cover and streamline a bird’s body.streamline a bird’s body.– Called Called flight feathersflight feathers if they if they

extend beyond the body.extend beyond the body. Down feathersDown feathers are soft and are soft and

have no hooks on barbules.have no hooks on barbules. Filoplume feathersFiloplume feathers are hair- are hair-

like – function unknown.like – function unknown. Powder-down feathersPowder-down feathers

disintegrate as they grow, disintegrate as they grow, releasing powder that aids in releasing powder that aids in waterproofing.waterproofing.

Feather structureFeather structure A contour feather has a long central shaft and a broad flat A contour feather has a long central shaft and a broad flat vanevane. .

The hollow base of the shaft (The hollow base of the shaft (quillquill) anchors the feather in a ) anchors the feather in a follicle under the surface of the skin. follicle under the surface of the skin.

The rest of the shaft, the The rest of the shaft, the rachisrachis, supports the vanes. Branching , supports the vanes. Branching off from the rachis are off from the rachis are barbsbarbs. Each barb has barbules projecting . Each barb has barbules projecting to either side that interlock with the barbules of adjacent barbs.to either side that interlock with the barbules of adjacent barbs.

Barbs and barbules form an interlocking, but flexible surface. Barbs and barbules form an interlocking, but flexible surface.

Form & Function – FeathersForm & Function – Feathers

Feathers are lightweight, Feathers are lightweight, yet tough, consisting of:yet tough, consisting of:– A hollow A hollow quillquill emerges emerges

from the skin.from the skin.– This becomes the This becomes the shaftshaft

which bears numerous which bears numerous barbsbarbs that form a flat, that form a flat, webbed surface, the webbed surface, the vanevane..

» Each barb contains many Each barb contains many barbulesbarbules..

Feathers

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.

Aftershaft

Ptarmigan

Form & Function – FeathersForm & Function – Feathers

When fully grown, feathers contain no living When fully grown, feathers contain no living cells– like mammalian hair.cells– like mammalian hair.

Birds Birds moltmolt to replace worn out feathers. to replace worn out feathers.– Usually feathers are discarded gradually to avoid Usually feathers are discarded gradually to avoid

bare spots.bare spots.– Flight feathers & tail feathers are lost in pairs to Flight feathers & tail feathers are lost in pairs to

maintain balance.maintain balance.– Many water birds lose all their primary feathers Many water birds lose all their primary feathers

at once and are grounded during the molt.at once and are grounded during the molt.

Form & Function – FeathersForm & Function – Feathers

Colors in birds may be Colors in birds may be pigmentarypigmentary or or structuralstructural..– Red, orange, & yellow are colored by pigments Red, orange, & yellow are colored by pigments

called called lipochromeslipochromes..– Black, brown, & gray are produced by the Black, brown, & gray are produced by the

pigment pigment melaninmelanin..– Blue is created structurally by the scattering of Blue is created structurally by the scattering of

shorter wavelengths of light by particles within shorter wavelengths of light by particles within the feather.the feather.

Adaptations for flightAdaptations for flight

In general, the avian skeleton has been In general, the avian skeleton has been lightenedlightened and and strengthenedstrengthened for flight. for flight.

This has been achieved by eliminating some This has been achieved by eliminating some structures and modifying others. structures and modifying others.

Adaptations for flightAdaptations for flight

Feathered wing.Feathered wing. Mass reduction Mass reduction

– Wrist bones reduced to two Wrist bones reduced to two – Bones hollow and supported by internal struts or Bones hollow and supported by internal struts or

spongy bone spongy bone – Reptilian tail lost. Fused tail bones (pygostyle) Reptilian tail lost. Fused tail bones (pygostyle)

support tail featherssupport tail feathers– Teeth lost. Skull and bill light but strong.Teeth lost. Skull and bill light but strong.

19.6

Figure 27.08

Adaptations for flightAdaptations for flight

Skeleton strengthened Skeleton strengthened – ribs have rear-facing uncinate processes that overlap and ribs have rear-facing uncinate processes that overlap and

strengthen walls of thoraxstrengthen walls of thorax– bones of wrist, pelvis fused bones of wrist, pelvis fused

Sternum or breastbone enlarged with a large keel (carina) for Sternum or breastbone enlarged with a large keel (carina) for attachment of massive flight muscles -- pectoralis and attachment of massive flight muscles -- pectoralis and supracoracoideus.supracoracoideus.

Fused hand bones support and maneuver primary flight Fused hand bones support and maneuver primary flight feathers. feathers.

Efficient lungs and powerful four-chambered heart power Efficient lungs and powerful four-chambered heart power flight.flight.

http://www.birdwatching-bliss.com/images/bird_skeleton.jpg

Furcula (in red)

http://upload.wikimedia.org/wikipedia/commons/6/6e/Furcula.png

Further skeletal modifications for Further skeletal modifications for flight: arm and fingersflight: arm and fingers

The wing is supported by the The wing is supported by the arm and finger bonesarm and finger bones. There are . There are three remaining fingers. The fused hand and finger bones provide three remaining fingers. The fused hand and finger bones provide strength and rigidity in the outer wing skeleton. strength and rigidity in the outer wing skeleton.

The primary feathers attach to the wrist and finger bones, while The primary feathers attach to the wrist and finger bones, while the secondaries attach to the radius and ulna. The first digit (the the secondaries attach to the radius and ulna. The first digit (the thumb if birds had one) controls the alula or bastard wing. thumb if birds had one) controls the alula or bastard wing.

The alula is a flap that is important in maintaining a smooth flow 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).of air over the wing at low speeds (it helps prevent stalling).

Bernoulli’s PrincipleBernoulli’s Principle

A bird’s wing is an airfoil and is cambered with a A bird’s wing is an airfoil and is cambered with a slightly convex upper surface and concave under slightly convex upper surface and concave under surface.surface.

Because air must travel further over the upper Because air must travel further over the upper surface of the wing than below it must travel faster surface of the wing than below it must travel faster and thus exerts less pressure above the wing than and thus exerts less pressure above the wing than it does below. it does below.

The increased pressure below generates lift, the The increased pressure below generates lift, the force which keeps the bird up. force which keeps the bird up.

19.13

Figure 27.16

Lift and thrustLift and thrust

In order to fly both horizontal thrust and vertical In order to fly both horizontal thrust and vertical lift are required.lift are required.

Thrust is mainly generated by the primary feathers Thrust is mainly generated by the primary feathers (the long ones at the end of the bird’s hand), which (the long ones at the end of the bird’s hand), which on the downstroke twist and acting like a propeller on the downstroke twist and acting like a propeller push the air backwards. push the air backwards.

Lift is mainly generated by the secondary feathers Lift is mainly generated by the secondary feathers (the inner portion of the wing), which form an (the inner portion of the wing), which form an airfoil. airfoil.

Flight - Wings are Specialized for Flight - Wings are Specialized for Particular Kinds of FlightParticular Kinds of Flight

Elliptical wingsElliptical wings are good for maneuvering in forests. are good for maneuvering in forests. High speed wingsHigh speed wings are used by birds that feed during flight or are used by birds that feed during flight or

that make long migrations.that make long migrations. Dynamic soaringDynamic soaring wingswings are used by oceanic birds that are used by oceanic birds that

exploit the reliable sea winds.exploit the reliable sea winds. High lift wingsHigh lift wings are found in predators that carry heavy loads. are found in predators that carry heavy loads.

Soaring over land with variable air currents.Soaring over land with variable air currents.

Arrangement of feathershttp://www.birdsnways.com/wisdom/imgs/wingsm.gif

Structure of hindlimbsStructure of hindlimbs

Legs joints and bonesLegs joints and bones. When looking at a . When looking at a birds leg what appears to be the knee is not. birds leg what appears to be the knee is not. It is the ankle. It is the ankle.

The knee joint is hidden by feathers. The The knee joint is hidden by feathers. The long bone leading from the toes is the long bone leading from the toes is the tarsometatarsus (from tarsals and tarsometatarsus (from tarsals and metatarsals) and above that is the tibiotarsusmetatarsals) and above that is the tibiotarsus

PerchingPerching

Being able to perch in trees was an early avian Being able to perch in trees was an early avian adaptation.adaptation.

The largest group of birds the Passerines The largest group of birds the Passerines (Passeriformes: perching birds) have four (Passeriformes: perching birds) have four moderately long toes, three facing forward and moderately long toes, three facing forward and one back.one back.

The tendons of the toes can lock the foot in a firm The tendons of the toes can lock the foot in a firm grip. Hence a sleeping bird does not fall off its grip. Hence a sleeping bird does not fall off its perch.perch.

Perching Song Thrush

Hopping and walkingHopping and walking

Most passerines hop and in fact most cannot Most passerines hop and in fact most cannot walk. However, some species that spend a walk. However, some species that spend a lot of time on the ground such as larks and lot of time on the ground such as larks and starlings can walk.starlings can walk.

In many other groups of ground-dwelling In many other groups of ground-dwelling birds (e.g. chickens and their relatives) birds (e.g. chickens and their relatives) however walking is used.however walking is used.

RunningRunning

A few birds are specialized for running and A few birds are specialized for running and possess long legs.possess long legs.

The most obvious are the ostrich and The most obvious are the ostrich and relatives. As is the case in horses the relatives. As is the case in horses the number of toes has been reduced to reduce number of toes has been reduced to reduce mass at the end of the limb. Ostriches have mass at the end of the limb. Ostriches have two toes and rheas three.two toes and rheas three.

Ostrich foot:

Climbing Climbing

Various birds including woodpeckers, Various birds including woodpeckers, nuthatches, treecreepers and woodcreepers nuthatches, treecreepers and woodcreepers climb up and down tree trunks. climb up and down tree trunks.

The feet in all cases are strong and the toes The feet in all cases are strong and the toes usually well curved and the tail is often usually well curved and the tail is often used as a brace to prop the body against the used as a brace to prop the body against the trunk.trunk.

Bennett’s Woodpecker

Montane Woodcreeper

SwimmingSwimming

A wide variety of birds swim and webbing A wide variety of birds swim and webbing of the front three toes has evolved of the front three toes has evolved independently at least four times and all independently at least four times and all four toes are connected by webbing in the four toes are connected by webbing in the pelicans and cormorants.pelicans and cormorants.

The other adaptation to increase surface The other adaptation to increase surface area for swimming is lobed toes.area for swimming is lobed toes.

Pelican Feet

Coot Feet

Avian lungAvian lung

The metabolic demands of flight are high and to The metabolic demands of flight are high and to meet these demands the avian lung must be much meet these demands the avian lung must be much more efficient than the mammalian lung.more efficient than the mammalian lung.

In birds the finest branches of the bronchi do not In birds the finest branches of the bronchi do not end is saclike alveoli. Instead they form tubelike end is saclike alveoli. Instead they form tubelike parabronchi through which air flows in only one parabronchi through which air flows in only one direction (in contrast to the tidal flow in direction (in contrast to the tidal flow in mammals). Because the air flow is one-way, gases mammals). Because the air flow is one-way, gases can be more efficiently exchanged and there is no can be more efficiently exchanged and there is no “dead air” in the lungs“dead air” in the lungs

Figure 27.12a

Avian lungsAvian lungs

The one-way flow of air is achieved by using a The one-way flow of air is achieved by using a system of air sacs and a two breath cycle.system of air sacs and a two breath cycle.

On inspiration a bolus of air flows down the On inspiration a bolus of air flows down the trachea to air sacs below the lung. On expiration trachea to air sacs below the lung. On expiration the air mass flows into the lung where gas the air mass flows into the lung where gas exchange takes place. exchange takes place.

With a second inspiration the air mass in the lung With a second inspiration the air mass in the lung flows into anterior airsacs and with a second flows into anterior airsacs and with a second expiration exits the body via the trachea. expiration exits the body via the trachea.

19.10

Figure 27.12b

Feeding and digestion: beaksFeeding and digestion: beaks

Birds are not the only organisms with beaks as Birds are not the only organisms with beaks as turtles and various dinosaurs possessed them too.turtles and various dinosaurs possessed them too.

However, birds have a tremendous diversity of However, birds have a tremendous diversity of beaks. beaks.

The beak is a birds principal tool for handling The beak is a birds principal tool for handling food and its size and shape determine the foods food and its size and shape determine the foods that can be eaten.that can be eaten.

BeaksBeaks

Dabbling ducks, such as mallards, have Dabbling ducks, such as mallards, have beaks that are good for straining water. beaks that are good for straining water. However, ducks such as mergansers have However, ducks such as mergansers have beaks that are serrated and help in grasping beaks that are serrated and help in grasping fish.fish.

Spoonbills and flamingoes possess bills Spoonbills and flamingoes possess bills similar to dabbling ducks that are good for similar to dabbling ducks that are good for sifting mud and water.sifting mud and water.

Flamingo beakhttp://spot.colorado.edu/~humphrey/fact%20sheets/flamingo_caribbean/images/flamingo_head.gif

BeaksBeaks

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

Ferrunginous Hawk

BeaksBeaks

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

Keel-billed Toucan

Great Hornbill:

BeaksBeaks

Among the more unusual beaks is the beak Among the more unusual beaks is the beak of the skimmer a relative of the gulls and of the skimmer a relative of the gulls and terns.terns.

In skimmers the lower mandible is laterally In skimmers the lower mandible is laterally compressed and the bird flies over the water compressed and the bird flies over the water with the lower mandible skimming through with the lower mandible skimming through the water. When the beak hits a fish, it the water. When the beak hits a fish, it snaps shut immediately on it.snaps shut immediately on it.

Black Skimmer

Feeding and digestionFeeding and digestion

Because birds lack teeth they can’t process food Because birds lack teeth they can’t process food much in the mouth so that is left up to the gastric much in the mouth so that is left up to the gastric system.system.

Birds can frequently gather food faster than it can Birds can frequently gather food faster than it can be processed. This food is usually stored in the be processed. This food is usually stored in the birds crop, an enlarged part of the esophagus. birds crop, an enlarged part of the esophagus.

The crop is also used to store food that will later The crop is also used to store food that will later be regurgitated to chicks.be regurgitated to chicks.

Bird digestive tract and other internal organs

StomachStomach

The stomach a birds possesses is dictated by The stomach a birds possesses is dictated by its diet. Birds that each high volumes of its diet. Birds that each high volumes of soft foods such as meat have extensible soft foods such as meat have extensible stomachs that can hold a lot of food.stomachs that can hold a lot of food.

Those that have to process tough foods such Those that have to process tough foods such as seeds and insects have much more as seeds and insects have much more muscular stomachs that help to grind the muscular stomachs that help to grind the food.food.

StomachStomach

A bird’s stomach has two parts the anterior A bird’s stomach has two parts the anterior glandular proventriculus and the posterior glandular proventriculus and the posterior gizzard. gizzard.

The proventriculus contains glands that The proventriculus contains glands that secrete digestive enzymes. In birds that secrete digestive enzymes. In birds that swallow whole foods such as fruits the swallow whole foods such as fruits the proventriculus is often very large.proventriculus is often very large.

GizzardGizzard

The gizzard’s main function is to The gizzard’s main function is to mechanically process food. mechanically process food.

The walls of the gizzard are thick and The walls of the gizzard are thick and muscular and the gizzard often contains muscular and the gizzard often contains small stones, which the birds swallow to small stones, which the birds swallow to assist in grinding the food. The gizzard assist in grinding the food. The gizzard thus fulfills the same role as the teeth in thus fulfills the same role as the teeth in mammals.mammals.

GizzardGizzard

The gizzard can exert significant pressure. The gizzard can exert significant pressure. For example, a turkey’s gizzard can process For example, a turkey’s gizzard can process two dozen walnuts in about four hours. two dozen walnuts in about four hours.

It can also crack nuts, some of which which It can also crack nuts, some of which which require 50-150kg of pressure to break.require 50-150kg of pressure to break.

IntestinesIntestines

The main site of chemical digestion is the The main site of chemical digestion is the intestine where enzymes break down the intestine where enzymes break down the food into small molecules that can be food into small molecules that can be absorbed across the intestinal wall.absorbed across the intestinal wall.

Seasonal changes in gut morphologySeasonal changes in gut morphology

Birds often change their diets over the course of a year and Birds often change their diets over the course of a year and gut morphology changes too. Insects are more easily and gut morphology changes too. Insects are more easily and quickly digested than plant food (e.g. berries). quickly digested than plant food (e.g. berries).

When starlings switch to eating more plant material in the When starlings switch to eating more plant material in the autumn their intestines increase in length by about 20% autumn their intestines increase in length by about 20% and decrease by a similar amount in spring when their diet and decrease by a similar amount in spring when their diet switches back to animal prey.switches back to animal prey.

Accompanying the morphological changes are changes in Accompanying the morphological changes are changes in the types and quantities of digestive enzymes produced the types and quantities of digestive enzymes produced tailored to match the composition of the diet.tailored to match the composition of the diet.

Reproduction

NestingNesting

PrecocialPrecocial young, such as young, such as ducks, water birds, fowl and ducks, water birds, fowl and quail are covered with down quail are covered with down when they hatch and can run when they hatch and can run or swim as soon as their or swim as soon as their down dries.down dries.– Most precocial young must Most precocial young must

still be cared for by the still be cared for by the parents for a time.parents for a time.

NestingNesting

AltricialAltricial young are naked and young are naked and unable to see or walk at unable to see or walk at hatching.hatching.– They must remain in the nest for They must remain in the nest for

a week or more.a week or more.– Parents must spend lots of time Parents must spend lots of time

& energy bringing food to & energy bringing food to hatchlings.hatchlings.

There is a continuum with the There is a continuum with the young of many species falling in young of many species falling in between the two extremes.between the two extremes.

Excretory SystemExcretory System

Urine is formed in large, paired Urine is formed in large, paired metanephric kidneys.metanephric kidneys.– There is no urinary bladder.There is no urinary bladder.– Nitrogenous wastes are secreted as Nitrogenous wastes are secreted as uric aciduric acid

rather than urea.rather than urea.– Bird kidneys can only concentrate solutes to 4-Bird kidneys can only concentrate solutes to 4-

8 times that of blood concentration.8 times that of blood concentration.

Excretory SystemExcretory System

Some birds, including Some birds, including marine birds, have a marine birds, have a salt gland to help rid salt gland to help rid the body of excess the body of excess salts.salts.– Salt solution is Salt solution is

excreted from the excreted from the nostrils.nostrils.

Nervous SystemNervous System

Birds have well Birds have well developed cerebral developed cerebral hemispheres, hemispheres, cerebellum (important cerebellum (important for coordinating for coordinating movement & balance), movement & balance), and optic lobes.and optic lobes.

SensesSenses

Birds usually have poor sense of smell & Birds usually have poor sense of smell & taste.taste.– Some, carnivores, waterfowl, flightless birds Some, carnivores, waterfowl, flightless birds

have well developed sense of smell & taste.have well developed sense of smell & taste. Birds have the keenest eyesight in the Birds have the keenest eyesight in the

animal kingdom and also very good animal kingdom and also very good hearing.hearing.– A hawk can clearly see a crouching rabbit 2.5 A hawk can clearly see a crouching rabbit 2.5

km awaykm away

Sensory systems: visionSensory systems: vision

Most birds have excellent vision and this is Most birds have excellent vision and this is reflected in the structure of the brain. There reflected in the structure of the brain. There are large optic lobes and the midbrain are large optic lobes and the midbrain which processes visual information is which processes visual information is enlarged.enlarged.

In contrast, in most birds olfaction is In contrast, in most birds olfaction is unimportant and the olfactory bulbs are unimportant and the olfactory bulbs are small.small.

VisionVision

Birds have very large eyes so much so that the brain is up Birds have very large eyes so much so that the brain is up and back in the skull to accommodate them.and back in the skull to accommodate them.

Bird eyes are similar in structure to those of other Bird eyes are similar in structure to those of other vertebrates, but the shape varies from a flattened sphere to vertebrates, but the shape varies from a flattened sphere to tube-like. tube-like.

The variation is shape appears to be a result of the The variation is shape appears to be a result of the difficulties of fitting an enlarged eye into a more modest difficulties of fitting an enlarged eye into a more modest sized skull. By altering eye shape birds such as owls have sized skull. By altering eye shape birds such as owls have avoided developing the disproportionately large heads they avoided developing the disproportionately large heads they would have required if the eye were spherical.would have required if the eye were spherical.

A unique feature of the avian eye is the presence of a A unique feature of the avian eye is the presence of a comb-like structure called the pecten.comb-like structure called the pecten.

The pecten arises from the rear of the eye close to where The pecten arises from the rear of the eye close to where the optic nerve exits the eye. the optic nerve exits the eye.

The function of the pecten remains unclear even after 200 The function of the pecten remains unclear even after 200 years of investigation. The organ’s large blood supply years of investigation. The organ’s large blood supply suggests it may provide nutrition to the retina and perhaps suggests it may provide nutrition to the retina and perhaps remove metabolic wastes from the vitreous humor.remove metabolic wastes from the vitreous humor.

VisionVision

A second interesting feature of avian eyes is A second interesting feature of avian eyes is the presence of colored oil droplets in cone the presence of colored oil droplets in cone cells.cells.

These act as filters absorbing certain These act as filters absorbing certain wavelengths of light and allowing others wavelengths of light and allowing others through, but their exact function remains through, but their exact function remains unclear.unclear.

HearingHearing

Birds have hearing that is comparable in Birds have hearing that is comparable in sensitivity to that of humans even though their sensitivity to that of humans even though their heads are much smaller.heads are much smaller.

However, they have proportionally much larger However, they have proportionally much larger tympanic membranes which enhances sensitivity tympanic membranes which enhances sensitivity to sound. In addition, the cochlea has about 10x to sound. In addition, the cochlea has about 10x as many hair cells per unit length than a as many hair cells per unit length than a mammalian cochlea does.mammalian cochlea does.

HearingHearing

Owls possess the most acute hearing among birds Owls possess the most acute hearing among birds (comparable to that of a cat) and can isolate (comparable to that of a cat) and can isolate sounds very accurately even in complete darkness.sounds very accurately even in complete darkness.

Owls possess a distinctive facial ruff of stiff Owls possess a distinctive facial ruff of stiff feathers that acts as a parabolic sound reflector, feathers that acts as a parabolic sound reflector, which focuses and amplifies sounds.which focuses and amplifies sounds.

Some ruffs are asymmetric and the ruff’s Some ruffs are asymmetric and the ruff’s asymmetry (as well as asymmetry in the vertical asymmetry (as well as asymmetry in the vertical placement of the ears) enhances the owls ability to placement of the ears) enhances the owls ability to isolate sounds in three dimensional space.isolate sounds in three dimensional space.

Barn owl

HearingHearing

The asymmetries in the ruff and ears cause delays The asymmetries in the ruff and ears cause delays in the time at which sounds reach each ear that can in the time at which sounds reach each ear that can be interpreted by the brain and used to identify be interpreted by the brain and used to identify precisely the source of a sound.precisely the source of a sound.

A barn owl’s ability to do this is so good that it A barn owl’s ability to do this is so good that it can isolate sound to within 1can isolate sound to within 1º in three dimensional º in three dimensional space. If you envisioned yourself surrounded by a space. If you envisioned yourself surrounded by a sphere with a radius approximately equal to your sphere with a radius approximately equal to your arm length 1º would be about the area covered by arm length 1º would be about the area covered by a fingertip.a fingertip.

OlfactionOlfaction

Most birds have a poorly developed sense Most birds have a poorly developed sense of smell, but a few groups do have a good of smell, but a few groups do have a good sense of smell. sense of smell.

These include kiwis which have their These include kiwis which have their nostrils at the end of the bill and use odor nostrils at the end of the bill and use odor cues to find prey when they probe in the cues to find prey when they probe in the earth. earth.

Brown Kiwi chick

OlfactionOlfaction

Other birds with a good sense of smell are Other birds with a good sense of smell are the various “tubenoses” the petrels, the various “tubenoses” the petrels, shearwaters and albatrosses, which are shearwaters and albatrosses, which are attracted to the scent of chummed fish and attracted to the scent of chummed fish and fish from a considerable distance.fish from a considerable distance.

In addition, turkey vultures have a well In addition, turkey vultures have a well developed sense of smell and can find even developed sense of smell and can find even covered carcasses very quickly.covered carcasses very quickly.