1. FEATHERS
The single most important diagnostic feature that allows us todetermine that an animal is a member of the Class Aves is thepresence of feathers. All birds have feathers. Feathers arehomologous to the scales of reptiles. There are even some fossilsthat appear to be a little bit intermediate between scales andfeathers although they are not really very good.
There are 2 fundamentally different types of feathers in terms oftheir structure. One of these are the feathers you are probablythe most familiar with and these are called flight & contourfeathers. They have a long central stiffening structure calledthe rachis that has branching off of it a large number of barbs.Those barbs have smaller branches off of them called barbules andin most birds the barbules are connected to one another bymicroscopic hooks that are called hamuli. The contour feathersare the ones that cover the body, the head, the back, the chest,and legs of the birds. The flight feathers include the obviouslarge feathers of the wings and the tail.
The second kind of feathers are down feathers and they do nothave a rachis or a hamuli. The down feathers are used forinsulation and not all birds have down feathers. Down feathersare found located underneath the contour feathers.
There's quite a bit of speculation about the evolution offeathers. Whether they originally evolved to be insulation orthey originally evolved to provide for flight or some otherfunction, perhaps associated with extensions of the limbs of theanimal. The oldest fossil of birds has the impressions offeathers in the matrix behind the skeleton and microscopicexamination of those impressions indicate that this very firstbird had feathers that were essentially indistinguishable fromthose of modern birds. So in that sense, we don't really knowvery much about the intermediate conditions that lead to theevolution of feathers. It's a little bit speculative as to whatthe function might have been.
2. SKELETON
The skeleton of birds shows a number of modifications for flight.They include the furculum - the technical name for the wishbone.The synsacrum - the large number of (frequently in theneighborhood of 20 or so) fused vertebrae which form a very solidbony connection between the axial skeleton, the vertebra and thepelvic girdle. The sternum on many birds, not all, has a carina -the mid ventral keel to which the flight muscles are attached.Even among contemporary living birds, the large flight wings onfor example the ostrich, do not have a carina on their sternumand they appear to have lost that as well as the power of flight.
The forelimb of a bird is fused so there's a very great reductionin the number of bones located in the carpus, metacarpus andphalanges. A very significant reduction in those bones. Also,most of the tail of modern birds is composed of feathers that areattached to a very contracted sequence of caudal vertebrae calledthe pygostyle.
3. SIZE RANGE
Now the size range in birds is interesting to compare with thesize range that we see in living mammals because it's quite a bitsmaller. The smallest birds and mammals are about the same size.The smallest adult body mass is a little over 2 grams, but thelargest living bird, which is the ostrich, weighs somethingaround 140 or 150 kilograms in a large adult male, whereas thelargest adult mammal is over 2,000 kilograms and that's of coursethe African elephant. There are larger birds known as fossils butthere are also larger mammals known as fossils. And so it seemspretty clear that whatever it is that sets the lower limit foradult body size in birds and mammals, it's probably the samething. The upper size limit is much greater in mammals probablybecause birds are all bipedal and they have to carry their entirebody mass just on their hind limbs and the very largest mammalsare not bipedal.
4. AVIAN ADAPTATIONS FOR FLIGHT
There are a whole series of adaptations that we see in birds thatcan be interpreted as being adaptations for flight. Flight isenergetically the most demanding form of locomotion that we seein vertebrate animals. The power required for an animal to fly ismuch greater than the power required to run or walk at the samespeed. The reason for that is the forces necessary to support thebody have to be generated aerodynamically when you are flying. Itdoesn't take a lot of energy to stand still and support your bodywith vertically oriented limbs.
So the 2 things that influence the adaptations for flight are: 1)weight reducing and 2) power promoting (increase or enhance themetabolic power available to fly).
Weight reducing:
1) Thin, hollow bones - The long bones birds like the ulna,radius and humerus and some of the long bones of the hind limbsare hollow. They have gas filled spaces in the middle of them andthose are connected by gas filled tubes to the animal'srespiratory system so that the pressure inside these gas filledspaces can equilibrate with the atmospheric pressure surroundingthe animal. There was a time in which there were speculationsthat respiratory exchange might take place because these gasfilled spaces in the bones were connected to the respiratorysystem but experimental evidence shows that there is nosignificant exchange of respiratory gases. There isn't anysignificant ventilation of these hollow spaces so they clearlyare simply a way to make a bone that's hollow and doesn't have asolid internal mass.
2) Feathers - Are very light. They are really quite remarkablylight for their strength.
3) No living birds have teeth but there are fossil birds that do.They have a beak in place of teeth which is composed of keratin(protein). The keratin beak of a bird covers a light-weight bonyjaw. The mandible (the lower jaw) of a bird is similar to thelower jaw of a reptile in that it is composed of a number ofdifferent bony elements in addition to a dentary and birds stillhave the same jaw joint, the articular quadrate jaw joint, thattheir reptilian ancestors have. Because birds don't have teeth inthat jaw, the mass of the jaw is quite a bit lower than it wouldbe if they had teeth.
4) All birds are oviparous. The best explanation for the reasonthat no species of birds evolved viviparity is that it savesweight. They don't have to carry the developing embryo around.They can lay lots of eggs and leave them in the nest and still beable to get around and not have to carry them during flight.
5) Birds have no bladder. So that's something else that theydon't have to carry around.
6) The female bird has only a single functional ovary at anygiven time. They have an undeveloped rudiment of the second ovaryand if you remove the primary one, this second one will develop.But in general, they develop only one and that seems to makesense as a way of reducing weight.
7) The testis of a male bird undergo very dramatic changes intheir size that are associated with their function. When a malebird if reproductively active, the testis is large and it isproducing sperm. It's producing testosterone and so forth andthen when it completes reproduction, the testis undergoes adramatic decrease in size. One term that we can associate thiswith is testicular recrudescence - refers to the growth of thetestis that occurs when they are increasing in size and gettingready for reproduction.
Power promoting adaptations:
1) Homeothermy - refers to having a stable body temperature.Birds generally have body temperatures that are higher thanmammals. Birds' body temperature range from 40 to 44 Celsius andeach species has it's own species specific value.
In order for an animal to have a constant body temperature, thereare a number of adaptations that it has to have. In vertebrateanimals, homeothermy requires endothermy (where the primarysource of heat is it's own metabolism). So we can say endothermyequals having a high metabolic rate.
2) Endothermy - high rate of heat production, oxygen consumption,food consumption and so forth. If we take a bird and reptile ofthe same exact body mass and put the reptile in a constanttemperature and heat it up so that it has the same bodytemperature as the bird, it would not bother the reptile at allin most cases and they will behaviorly regulate their bodytemperature in the same range. So we're comparing equal masses oftissue at the same temperature. We know that temperatureinfluences the rate of chemical reactions. We make thosecomparisons when the animals are both at rest so we get rid ofany influence of exercise. The bird will still have a metabolicrate that is about 6 times that of the reptile. It will beconsuming oxygen 6 times as fast and producing 6 times as manycalories per minute. That's what we mean by a high metabolicrate. The same general comparison would hold for mammals by theway.
- Feathers under endothermy as an adaptation that birds havethat's part of being an endotherm. The feathers of birds are abetter form of insulation than fur and they need a system ofcooling in order to be a homeotherm. So birds have a mechanism ofcooling that's similar to panting called gular flutter in whichthe gular region (area underneath the jaw) is pumped in and outand air gets pulled into the mouth. Water evaporates off of thelining of the mouth and that cools the skin down. Then the bloodgoing through the skin gets cooled down. You also have to have acentral controller - an area in the brain that measures bodytemperature, turns on the cooler, turns on the heater and soforth. Having a high resting metabolic rate probably also givesthe animal the capacity to have a high metabolic rate duringexercise and so that's how endothermy could be seen as being anadaptation to enhance power all by itself. Independent of it'scontribution to having a stable body temperature.
- Efficient lungs are under endothermy as well. Efficiency inthis sense, technically, should be a unitless number. It shouldbe a ratio of 2 things that are measured in the same units. So wecan define the efficiency of the birds' lungs as being the oxygenextracted divided by the total oxygen passing through the lungs.The fraction of the oxygen passing through the lungs of the birdmanages to extract and the bird's lung is much more efficientthan a mammal's lung. A bird's lung continue to obtain oxygenunder conditions where a mammal is in really bad shape and a goodexample of that would be at high altitudes. The reason why birdshave more efficient lungs is that they don't have a tidal system(mammals do). They have what's called a cross-current lung orexchanger. Cross-current means blood moving in one direction andgas moving in the other and part of the reason that birds areable to have a cross-current exchanger rather than a tidal systemis that they have air capillaries through which the gas involvedin respiratory exchange actually flows through the lungs, flowsfrom air sacs in the back end of the animal's body into air sacsat the front end of the animal's body and because it's flowingstraight through there is no volume of gas that remains in thelungs at the end of an exhalation.
In fact, if you watch a bird breathe, it looks very much like amammal breathing. You see the chest going in and out like amammal breathing but the lungs of a bird do not change theirvolume. Unlike a mammalian lung that's like a balloon thatexpands and contracts, what you see expanding are blindconnective tissue bags called air sacs that are located in theabdomen and other regions of the animal's body. If you can putsome kind of a gas marker into the volume of air that the animalinhaled, you'd find that marker coming right back out in the nextbreath of a mammal, reptile or amphibian. If you do that with abird, it does not come out in the next breath. It takes 2complete respiratory cycles for a given volume of gas to comeback out of a bird's lung. During the first part, it goes downinto the air sacs. And during the second part of the respiratorycycle, it passes through the lungs.
I'm not going to go into any more detail of how the respirationworks in the bird's lung but it is fundamentally the mostdifferent of all the physiological systems that we compare birdsand mammals with. There are many, many ways in which they arevery similar: they are endotherms, they have 4-chambered hearts,and their kidneys work in a somewhat similar fashion. The biggestsingle difference between birds and mammals is in the way inwhich their lungs work and that is reflected in the greaterefficiency of a bird's lung.
- 4 chambered heart is part of endothermy. A bird's heart isabout twice as large as the heart of a mammal of the same bodysize (2 x mammal). Compare a little mouse and a little bird withthe same body size. The bird's heart is going to be twice aslarge.
Those are all power enhancing adaptations that are part ofendothermy which can be interpreted as an adaptation to enhancingpower.
Another power enhancing/promoting feature of birds in generalwhich isn't part of endothermy is that they have very largepectoral muscles. These are the breast muscles, the ones that areresponsible for the down stroke, which is in most birds the majorpower generating stroke of flight. The pectoral muscles in apigeon for example comprise 30% of the entire body weight of apigeon. That's a really remarkable number for a single muscle tobe that large a fraction of the animal's total body weight.
So many adaptations of birds can be interpreted as contributingto flight, either by reducing weight or by enhancing power. Thereare other interpretations possible as well.
What birds do for digestion of food is they have a specializationof their esophagus called the gizzard which is the muscularstructure that has a hard keratinous grinding surface as theyswallow stones down into that gizzard and the mechanicalbreakdown of the food which has the same purpose as masticationin a mammal is performed by the gizzard so they really have sortof evolved the gizzard because they use their beak as a means ofacquiring food. Birds that probe for food have long narrow beaksand I'll talk about that later. In many cases the beaks of birdsare specialized for acquiring food but it still can be a weightsaving adaptation.
5. AVIAN BRAIN
One way in which birds differ very greatly from mammals is intheir brain. Birds seem to be profoundly stupid by human terms.The bird's brain is, compared to it's body size, as big as amammal's brain. It does not have a neopallium because birds havealmost no sense of smell at all. The only birds that have anysense of smell are vultures. Birds have a very good sense ofhearing but their brain has enlarged from the reptilian conditionin a fundamentally different way than that of mammals. They havean enlarged area called the corpus striatum where informationfrom the eyes is processed, in a manner very similar to whathappens in the neopallium of mammals.
6. COMPLEX BEHAVIOR
Birds have kind of followed a different route towards achievingsome incredible complex behaviors from what we see in mammals.And I like to draw the analogy between different kinds ofcomputers. You know you have an Apple or an IBM computer, you'redealing with a computer that is a programmable computer. You cantype in some code or you can transfer a program into it off of afloppy disk and that computer has the ability to perform a widevariety of different functions and if you change a program it'llperform a different function. So in that sense, those computersare like the brains of mammals. They have the ability to modifytheir behavior on the basis of information that they get put intothe them. But you may also have a hand calculator that'snon-programmable and that is what we would call a hard-wirecomputer. It has certain capabilities that it can do very well.It does them exactly the same way every time you push the buttonand it's a different way to build a computer, one that'shard-wire. The bird's brain is like a hard-wire computer.
Birds have some incredibly complex behaviors that they are bornknowing how to do. Migratory behavior is one of them. There arespecies of ducks for example that nest on the tundra way up abovethe Arctic circle in the summer and the breeding season up thereis short, the weather is somewhat unpredictable and after thebaby birds have been hatched and are largely grown, the adultbirds feed madly trying to get enough fat into their adiposetissue deposit so that they can take off on migration. When theadult birds are sufficiently fattened, they will form a hugeflock and they will all fly off. They will fly perhaps 5,000 or6,000 miles. If they are flying at night they can use the starsas a compass. If they are flying in the day time they can use thesun as a compass and to compensate for the movement of the sun,they will follow river valleys and coasts. They will get to thecoast of Texas and know that they've got hundreds of miles to flyacross open ocean and will load up with food and water beforethey take off on that long flight. They will wait until weatherfronts have passed through so that they don't get stuck in badweather over the ocean and then they take off on their migration.They get down into Central South America some place and they goto a particular habitat/location. They left all the babiesbehind. The babies take another 2 or 3 weeks to fatten up andthey take off on their migration and do the exact same thing.They know that they are supposed to fly south, they know whatdirection is south, they know how to tell the movement of thesun, they know how to tell the movement of the stars, they knowhow to anticipate the Gulf of Mexico, and they have all theinformation that they need to take on that entire migration. Theyeven know what it's going to look like when they get there, whatis the right habitat to stop in, and they show up in the samehabitat that their parents stopped in a couple of weeks beforeand there are no adult birds along on that migration that areleading them on the way. So they know how to do that. There arespecies of birds that can cross literally thousands of miles ofopen ocean.
There are some amazing studies done on homing in albatrosses thatnests on Mid-Way Island which is located out in the middle of thePacific Ocean thousands of miles from land. These great bigbirds, also called "goony bird", nest on that island.They have a very long incubation period and the male and femaleboth sit on the eggs. Some scientists waited until one of thebirds had come in to take over the nesting (normally the othermate would be gone for a couple of weeks) and they grabbed themates that were about to leave and put them on a Navy transportplane to the Philippines, to San Diego, to Seattle. To placesthat are outside of the normal range where these birds have everbeen in their whole entire lives; in different directions fromMid-Way Island. When they turned the birds loose, they all gotback to where they belong, homing across thousands of miles oftrackless ocean, able to tell that they were east of Mid-Way orwest of Mid-Way and found that little tiny spot of land in themiddle of the ocean that human navigators with radar andsatellite navigation equipment have failed to find. The birdswere able to find the islands no problem and nobody knows howthey did it.
So birds really do perform some incredible behaviors that arehard-wired into their brains and because they adopted that as anevolutionary strategy of building these behaviors into theirbrains, they don't have much ability to modify their behaviors.Birds that sing particular songs as part of courtship andterritoriality are born knowing most of that song and then learnthe rest of it from their parent who they listen to singing whilethey are sitting in the nest defending the territory. Thesealbatrosses on Mid-Way Island, their nesting behavior is reallyremarkable in that there may be a thousand nests and they canfind their own eggs every time. They can recognize differences innests. I mean little tiny shallow scraped out areas of sand.
8. REPRODUCTION
Courtship in birds is another example of complex behavior. Itinvolves territoriality. Many times, male birds will stake outterritories and females will choose males on the basis of thequality of their territory which may involve nest building.Sometimes the male birds will build a whole bunch of nests andthe females will come in and fly around and sort of check out allthe nests to see who has the best place before they choose tomate with them. All sorts of interesting postural displays thatinvolve sounds and displays of brightly colored patches offeathers are involved in courtship.
Some bird singing is courtship singing and a lot of it isterritorial defense. In courtship, female birds will be broughtabout half way to reproductive condition by changes in the daylength but in most cases or at least in many cases in temperateregions, the courtship of the male is necessary to bring thefemale all the way to reproductive status. So she will sort of beprimed to the condition where she's sort of interested incourtship behavior but she has to be fairly extensively courtedby a male before she will be actually physiologically ready tobreed.
Fertilization in birds is internal. All amniotes have internalfertilization and birds usually obtain fertilization by means ofa cloacal kiss. There are a few species of birds like ducks thatseem to have very small intromittent organs but most birdsutilize just a cloacal kiss.
Oviparous - They are all oviparous laying amniotic eggs with aheavy calcareous shell (calcium carbonate formed).
Incubation is found in almost all species of birds. In manycases, probably in half of the species, incubation involves boththe male and the female taking turns incubating the eggs. Theincubation period is the length of time that the egg has to bekept warm for development to be completed. That tends to varywith egg size. In general, larger eggs take longer to developthan small eggs. But there's a lot of ecological variability inthat. Usually species that nest in very safely protectedlocations like the albatross, nest on an islands where there areno natural predators and those species will have very longincubation periods.
The egg of the same size laid by a duck or something like that onthe mainland where there's predators and other pressures tocomplete reproduction will have a much shorter incubation period.So in general, the incubation period varies with egg size butthere is a lot of ecological variability.
Egg temperatures during incubation range from about 35 to 38Celsius. So egg temperatures are usually several degrees lowerthan the body temperature in an adult that's sitting on top ofit. And these are species specific. If you are a couple ofdegrees off, the hatchlings may be malformed.
CONDITION OF YOUNG AT HATCHING:
At birth, birds range from precocial to altricial.
Precocial species are things like ducks and chickens. Baby comesout all covered with down and is able to locomote; run around.The down feathers are there for insulation so it's able tomaintain a fairly stable body temperature. It has it's eyes openso the sensory systems are functioning. It's legs are working andit's able to run around and it finds it's food.
Altricial species are things like robins, sparrows and blue jays.These birds, when the babies are born, are really helpless. Theyhave almost no down on them so they cannot regulate their bodytemperature and their parents have to sit on them to keep themwarm. Their eyes are frequently closed so their sensory systemsare not completely developed. They just kind of sit there in thenest and scream for food which the parents bring to them.
These are sort of extreme conditions and there are many speciesthat fall in between the 2. Like hawks for example are coveredwith down but they stay in their nest. So these animals are sortof intermediate between the precocial and altricial conditions.
Parental care again have a lot of variabilities. Usually, birdshave fairly extensive parental care, keeping the babies warm.Even in the precocial species, the parents will stay with thebabies, usually warn them of approaching predators, point outwhat food looks like, and keep them warm at night when it'sparticularly cold. And of course in the altricial species,there's a lot of parental care.