ZOO 138, Friday, February 28, 1997, 12:00 p.m.
The feather is the single most definitive diagnostic characteristicfor the class of birds. And this is the first time we have encounteredthis kind of situation. In the past there have been a suite of charactersthat define a class. This is the case in reptiles or amphibians. If you have feathersyou are bird; if you don't have feathers you are not a bird. It'sas simple as that.
There are many other generalizations that I will make aboutthe anatomy and physiology and so forth of the class, but feathersare the distinguishing characteristics. And, you know, like fur,feathers or homologous to scales, that is in an evolutionary sincefeathers have evolved from scales, but fur did not. Fur beinganalogous to a mammalian feature.
There are 2 different types of feathers in terms of their anatomy.These are called "flight" and "contour" feathers.
Fight and contour feathers have a whole series of anatomicalfeatures in common. Flight feathers are the ones that make upmost of the surface of the wing. They are called "primary"and "secondary" feathers. And the ones that are on thewrist and further out are the primary feathers. The feathers onthe ulna are the secondary feathers, but those are all flightfeathers.
And also the surface -- the surface of the tail is composedof flight feathers as well. Contours feathers cover the rest thebody of the animal, the head and the chest and down the legs alittle bit.
And they have this central stiffening structured called a "rachis."And branching off both sides of rachis are barbs. And if you lookat the Illustrated Notes you can see a series of progressivelyhigher magnifications coming off the barbs are barbules.
And then finally, there are these little tiny hooks called thehamulus. Hamulus is the singular form; hamuli is the plural form.Those are hook-like structures that are really responsible forthe ability of a feather to maintain a fairly rigid flat surfacestructure. They connect the various barbs and barbules to oneanother.
Now, at sometime or another most people have played around witha bird's feather. You can pull it apart and then sort of mushon it and preen it the way a bird would. You can get it to knitit back together again. That's a very important feature.
That feature is the result of a presence of a hamuli, theselittle hooks. When you pull it apart you're separating the hooksfrom the barbs, and then when you preen it and put it back togetherthe hooks are reconnecting again.
So you can see that makes for this to be something that givesyou a pretty stiff broad surface area, which you need in orderfor it to make a flight surface of the wing. So if the bird happensto brush its wing against something, the feathers don't becomepermanently damaged by that. They do eventually get frayed andare replaced.
The second type of feather that we find on these animals isa called a "down feather." And the down feathers donot have a rachis. They don't have a hamuli. What they are likeis a whole series of barbs and barbules arranged in kind of likea crown. And you can't see it in that picture there, but basicallywhat a down feather looks like is a base with a whole series ofbarbs that are all stick up, you know, maybe like the feathersof a headdress in a native America headdress.
But those don't have hamuli connecting them together. So theyare really soft and mushy and tend to have a very, you know, morephysical type of shape such as what you see illustrated on thepage here. And down feathers are used for insulation. Insulation.
There is quite the controversy among evolutionary biologistsas to whether the first feathers were flight feathers or downfeathers. In other words, were they originally evolved for flightor for insulation. Because down is the primary insulating devicein these animals not the flight feathers. Flight feathers areless effective for insulation.
Feathers are molted they have a programmed time of the year,usually twice a year. The animal will lose feathers in an organizedfashion. Sometimes, if you see a wild bird or even a caged birdin captivity, you can see sort of a line somewhere along the bodywhere there is an obvious difference in sort of freshness of thecolor and the freshness of the feathers.
What happens is the feathers fall out progressively and newones grow in over the entire surface of the animal's body. Andthe same thing is true of the flight feathers and the wing feathers.Sometimes if you watch a bird flying overhead, you see its wingsspread out, you will see symmetrical holes in the wings whereit has lost one of its primary or secondary flight feathers.
So it will lose one, grow in a replacement and then lose thenext one adjacent to it. So it maintains a fairly complete surfacefor flight while replacing the ones that are worn out, and frequentlyin a coordinated time to coincide with migration.
For example, a bird might grow a new flight feather in anticipationof migration or it might grow a new set of contour feathers inparticular, any bright colors as part of the preparation formating season. That's most likely in the male bird. They mightgrow a whole new set of down feathers in the fall just beforethe winter comes along. They do these things in ways that makesense in an evolutionary sense.
There are some really interesting examples of birds that aremigratory, like ducks that lose all of their flight feathers once,rather than messing around with the business of gradual replacementof feathers one at a time. They lose all their flight feathersand they're incapable of flight for a month or so.
And they will also usually -- particularly in species that arevery brightly colored, because being brightly colored increasesthe likelihood of a predator being able to find you, these birdswill grow a very cryptic or camouflage set of feathers prior tolosing all of the flight feathers. So they have a very short timewhen they have camouflage feathers when they have lost their flightfeathers.
Now, the skeleton of birds exhibits a whole series of modificationsfor flight. You have looked at some of these in lab. I'll justquickly run through them. You know that a bird has a "furculum."That's the technical name for wishbone, which is composed of acouple of clavicles an inner clavicle -- and they have synsacrum, which is formed by the fusion of something in the neighborhoodof 20 individual vertebrae.
That's a much larger skeletal structure than we see in mammals.Mammals of at the most maybe 3 vertebrae that are fused. And itserves as a point of attachment of the pelvic girdle. But birdshave a much larger synsacrum that forms a larger solid rigid bonystructure for the attachment of the pelvic girdle. And, of course,that's where the landing gear, the hind limbs of the bird areattached to the pelvic girdle.
And the pelvic girdle of a bird does not have pubic synthesis.Remember in looking at the skeleton of the cat you saw the pubicsynthesis, where the 2 halves of the girdles meet at the midline.There is also a synthesis between the two ischia and the bird'spelvic girdle, the 2 halves are separate from one another. Andso they must have a much larger rigid sacrum to which they canbe attached because. They don't have the additional strength thatwould come from having that fusion in the midline ventrally.
Most birds have a carina. The carina is sometimes called thekeel on the sternum. It's that plate that sticks out ventrallyin the midline and serves as a point of attachment for the flightmuscles. The pectoral are the big muscles that are primarily responsiblefor powering flight. And the carina serves as a point of attachmentof that.
That's if the bird's skeleton -- even a bat, which is a mammalcapable of flight has a tiny little carina on its sternum to whichflight muscles are attached and there is extensive reduction inthe bones of the wrist and the hand, down to just three or fourbones.
Think of all the bones that you have in your wrist and yourcarpus and metacarpals and phalanges. It's reduced to three orfour bones in a bird and they serve as a point of attachment forsome of the flight feathers. So we see a whole series of osteologicalspecializations in birds for flight as well.
The size range in birds is narrower than what we see in mammals.The smallest birds weigh around 2.2 grams, maybe 2.1, 2.3. That'san adult body mass, the humming bird, for example, a little tinyhumming bird found in Central America. There is also a Swift (spelling)which is little tiny bird found in Asia that comes in that bodymass.
And interesting enough, that's about the same size as the smallestadult body mass of all the mammals. In fact, that may not be acoincidence. Maybe that's just about smallest you can make a vertebratehomeotherm and endotherm, because that's an incredibly tiny littleanimal and yet it has the ability to reproduce. It has all thesame internal anatomy and everything that all the other vertebrateshave.
But that's about the smallest adult package that you will findanywhere. The maximum body size in birds -- among living birdsis around a 150 kilograms. That is the adult male Ostrich. Thereare some fossil birds that are about three times as big as thatfound on several different continents.
There is one here in North America that is even three timesthat. That's much smaller than the largest body mass among mammals.I'm talking about terrestrial, and I'm not talking about the bluewhale, just about living terrestrial mammals. The same minimumbody size, but somewhere in the neighborhood of around 2000 kilogramsis the largest. And that's the body mass of an adult terrestrialmammal.
Mammals come in a wider range of sizes, but it's really differenton the high end, not on the low end.
Now, that may be a result of the fact that birds are all bipedal.And the largest mammals are quadrapedal. 2 legged versus 4 legged.Although, if you stop to think about it you might think of a reasonwhy that argument doesn't hold any water. But I'll let you thinkabout that for a minute.
What's wrong with that argument or explanation that a reasonmammals are bigger is because they are quadrapedal.
STUDENT: The blue whale --
INSTRUCTOR: We're only talking about terrestrial mammals, we'renot talking about the blue whale which is a lot bigger but doesn'thave to support its body weight in water. I'm sticking to terrestrialanimals.
STUDENT: Small ones that are --
INSTRUCTOR: Well, there are small ones. I'll just say the reasonthe biggest mammal is, you know, at least twice as big, maybethree times as big or 4 times as big as the biggest bird is becausemammals are quadrapedal. And they can use 4 limbs to support thatbody mass, where birds are bipedal. There is a fallacy in thatargument.
STUDENT: Because of--
INSTRUCTOR: It could be some other explanation. It might verywell be the size of the animal has a big impact on its whole biology.It has to get enough food to make a living. You can't make a livingon seeds if you you're as big as elephant. You have to eat.
There is probably another explanation since the one I providedyou is fallacious.
STUDENT: The only thing I can think of may be the competitionwith the animals. They can't get that large. Even though wehave seen in the past dinosaurs can be quite large and be bipedal.
INSTRUCTOR: To me, that fallacy, the argument is the one youjust cited. And that tyrannosaurus rex would have had a hard timebuying that explanation.
STUDENT: I'm thinking of like the duck bill -- you know, theduck bill dinosaurs.
INSTRUCTOR: Any of those were bipedal and they were substantiallylarger than an elk. So that's the fallacy. Everybody understandthat the fallacy is that you can be a lot bigger than that andstill be bipedal. That can't the reason birds are smaller. Andit probably has something to do with the other aspects of theirbiology, whether it is competition with mammals or what you aregoing to feed on if you are that big.
Now, when we look at the overall biology of birds, we can seethat some of the adaptations in birds seem to make sense. And,again, they may become fallacy here. But it seems to make sensein terms of reducing weight because of the mode of locomotion,but not all birds, most.
Flight is the most engergetically, the most demanding form oflocomotion among all animals. It takes more energy to get up inthe air and fly around than it does to run. For example, whereyou have nice direct support of your limbs and contact withthe ground, that takes more energy than it does to swim, wherethe mass of the animal is basically supported by buoyant forcesassociated with the difference in the density of the medium.
But to fly takes a huge amount of power and you can minimizethe power requirements if you reduce the weight. So some of theadaptations in birds we can see as being ways to reduce weightand some of the adaptations we can see as being a means of enhancingpower, that is metabolic rate required for flight.
One of the reducing adaptations in birds might include the hollowbones. In other words, in lab you have looked at the bones ofmammals and birds. You may or may not have noticed but the interiorof a mammal's bone -- not only is there calcium carbonate or themineral that makes up most the mass of the bone, it is much heavierand thicker. But mammals have a marrow cavity that's filled withtissue that is responsible for the production of red blood cells.
Whereas birds' bones are hollow. They have a air filled spacein the middle -- in particular of their long bones, but many evensmaller bones have an air filled space in the middle of the bone,and that's going to reduce the weight of the bone substantially.
In fact, there is a really neat bird that you can see alongthe coast down in Mexico called a Fridget (spelling) bird. Ithas a huge long wingspan. It looks like a high-tech glider plane.It has a real long narrow wing. It's a glider bird. And it's apirate. It's harasses seagulls and other birds and steals foodfrom them.
So it glides on thermals up drafts of winds, and so this isan animal that has been subjected to a very intense selectionfor reducing the weight of its skeleton so it that it can do alot of gliding in very light winds and saves energy and looksfor somebody to steal its lunch from. And the Fridget birds' featherscovering its body in forming the aerodynamic serves its tail andwings way more than its skeleton.
The feathers of a Fridget bird weigh more than the skeletonof the bird. So, in fact, the skeleton of the bird is a very,very light structure. And that may be a means of reducing theweight that the animal has when it goes into flight.
Feathers themselves are very, very strong for their weight andform. Not only is it an excellent form of insulation, but thisaerodynamic surface of the wings and tail, the feathers appearto have evolved in response to the selective pressures that wouldreduce their weight.
Birds have no teeth as you probably know, at least modern birds.You see when we talk about the classification of the natural historythe birds next week, that is the primative birds had teeth, butthey have lost them in an evolutionary sense. No modern birdshave teeth. They have a beak form of keratin, which is a proteinthat your fingernails and your cat's claws and so forth are madeof.
It's a good strong structure protein the vertebrate animalsused when they want to make something that's quite durable andgrowing. The beak of a bird grows. So they have evolved a beakas a means of reducing weight.
They don't have to have all of the mass of the teeth themselves,but they also can have a much lighter dentary bone and other benefitsof a comprised lower jaw of a bird, which are hidden beneath thebeak of the bird. Inside the beak of the bird really is a verylight bone, so that has reduced weight.
STUDENT: May I ask you a question? If they don't have teeth,does the weight of the teeth -- would the weight of the teethequal the weight of the gizzard, which many birds have. A lotof them use -- you know, like chickens have a very big gizzardto grind up their food?
INSTRUCTOR: That's a very good question. It's a little bit outof sequence, but one of the things that I'll be telling you aboutwhen I talk about the classification and natural history of birdsis that the birds primarily, not exclusive, but primarily birdsuse their beak to acquire their food.
So they might have a long narrow beak for probing in holes toget the food. They do not chew their food. They have not evolvedmastication in the sense of using jaws and teeth to grind up theirfood.
But as Mary Jane says, they have an organ which was a specializationof their esophagus, which we call the gizzard which performs thatsame purpose. Because one of their requirements is high energyrequirements, they need to increase the surface area to act ondigestive enzymes. The gizzard performs the same function.
And question then is does the mass of the gizzard equal themass of the teeth? And I would guess that it may not be dramaticallydifferent. So that's one reason for criticizing my assertion thatbirds lost teeth as an adaptation for reducing weight.
Because if they have to grow a gizzard to replace it or growa gizzard, an alternative means of doing the same thing, theymay not have saved any weight if it weighs the same.
It might have something to do with the distribution of weightbecause now your weight is back down in the middle right undernear the center mass rather than way out in the front end of theanimal, which might make it a little easier to fly. So that'sanother reason for criticizing this assertion as well.
STUDENT: Do we know whether any prehistoric birds also had agizzard?
INSTRUCTOR: There is no way to know, although that may not betrue. Because a gizzard -- and even in dinosaurs there are theseround rocks that are found in the area of the middle of the animalof the fossilized skeleton. The animal which is assumed to begrinding, you know, the rocks that were in the gizzard when itwas being used for grinding so, maybe, actually it would be possibleto tell that.
STUDENT: So they lost the teeth, they didn't necessarily haveto grow the gizzard, it was already there?
INSTRUCTOR: If you were using a gizzard to grind your food thenyou didn't need teeth. Was there another argument, another criticismof the assertion that birds have a beak as means of a reducingweight?
See that's the way that you can use your knowledge of the naturalhistory of vertebrates, is you might say, well, are there anyother animals that have beaks, and do they lose their beak tosave weight?
What are the other 2 other groups of vertebrates of beaks, oneextant and one fossil. Turtles have beaks, they don't look likethey are under a lot of selective pressure to reduce weight.
You may not have noticed it, but anybody know? The Ornithischiandinosaurs, like triceratops, have beaks. They weren't under pressureto reduce weight at the front end of the body or the end of thebody. The classical story, well, birds have beaks to reduce weightmay not hold up to critical examination either.
STUDENT: Also, you have bats who are flying mammals who don'thave beaks.
INSTRUCTOR: Very good. Bats. That's another way you can useyour knowledge of natural history to question these stories. Batshave teeth.
STUDENT: Bats have short necks and many birds that fly havelong necks.
INSTRUCTOR: Distribution of weight. It's a distribution thing.But then you have to ask whether or not birds that lost theirteeth have long necks.
STUDENT: Well, did -- have a long neck?
INSTRUCTOR: Probably longer than a bat. That's good. That'sthe kinds of thing we can do with this information that we'reacquiring here.
Birds are also the only major group of vertebrates that areexclusively oviparous.
And the even though oviparity is a primative condition amongvertebrate animals, the fact that there are more than a dozendifferent evolutions of oviparity among reptiles and thousandsof species of fish that evolved oviparity, and the majority ofmammals are and even amphibians that evolved it. Within that context,the fact that all roughly 8,000 or 9,000 species of birds areexclusively oviparous begins to look like well maybe this is away of reducing adaptations.
Maybe it prevents them from having to cart those critters aroundduring development. If they can deposit them in the nest someplaceand sit on them and keep them warm. Obviously, bats are viviparous.And what's the lit litter size in bats, it's one. You can't getany smaller than that. There may be some tradeoffs there, birdscan have a clutch, we call it. Have a couple dozen at one time.
So there may be some tradeoffs there. That may be a valid one.
Birds have no bladder. And that means something else that theyare not having to carry around. Although, there may be some physiologicalexplanations for that as well.
But there are some interesting things that birds do like birds-- female birds have only one functional ovary at any given time.And that is really a weird thing. They have that during embryogenesis,the female bird develops the embryonic primordia of 2 normal vertebratefemale reproduction active tracts, both horns of the uterus, bothovaries. But at some point during maturation, only one of thosewill a mature.
And that probably is a legitimate way of reducing adaptation-- weight reducing adaptation.
STUDENT: Does that apply even to the birds that are flightless?
INSTRUCTOR: Yes, it's my understanding that it does. It couldbe a permanent feature because the current flightless birds aredescendants from birds that could fly, so it could be a primativefeature.
Now, power promoting adaptations. I'm not going to have timeto go into it, remember we I argued that homeothermy was a solutionto the problem of variations in environmental temperature. Andthat's true, however, if you have an animal that is flying becauseit is a engergetically such is a demanding form of locomotion.It has the potential to raise the body temperature of the animal.
So, for example, when we look at insects like months and beesand butterflies they are also homeothermic because of the hugeheat production that occurs during flight. And so in a sense homeothermywould also be an adaptation to flight of a way of preventing excessivegains in body temperature while the animal is flying.
And endothermy, that is having a high metabolic rate, can alsobe not only a mechanism of achieving homeothermy, but also bea way of providing an animal with very high power output neededfor flight.
In other words, when we look at the maximum rate of sustainableenergy utilization in all vertebrate animals, it's almost alwaysabout five or ten times their basal rate.
All right. There are a few interesting very athletic organismslike horses and humans that can get up to 20 times their restingor basal rate. But in most cases it's only five or ten times.And that's true of amphibians, reptiles, as well birds. And soby having a higher basal rate you have a higher maximum rate.And so endothermy maybe an adaptation promoting power necessaryfor flight.
Of course, insulation that is feathers, is a part of homeothermy.
Birds have a highly efficient lung. Now, "efficiency"is a word that gets misused a lot of the time. Technically froman engineering point of view, efficiency is the ratio of two thingswhich have the same units. So you have -- in this case, what I'mtalking about is birds can extract a much larger fraction of oxygenin the air that they breathe than can a mammal.
And when we look at all the aspects and anatomy and physiologyof birds, animals, mammals, the 2 vertebral endothermic classes,we see many many similarities. Many similarities. The biggestevident single difference between the birds and mammals in myopinion is in the function of the lung.
The lungs of the bird are fundamentally different from yourlungs and the lungs of other mammals and the lungs of all othervertebrate animals. Birds have a lung where the lung itself isnot elastic. When you see a bird's thoracic cavity expand duringinhalation, it's not because the lung is getting bigger. Thatair is going into air sacs which are pouch-like structures thatare attached to the trachea of the animal.
And then the air passes through what are called air capillaries.They are tubular structures. And the air passes through the lungsin a unidirectional manner. In fact, you know, the good air comesin and the air goes out version of respiration.
If we put a marker gas in the air that you are inhaling we couldput helium, if you sit there breathing and we had a way of monitoringthe gas contents of the air that you exhaled, if we put a littlepuff of helium into the air you're breathing, that helium wouldappear in the very next exhalation. But if we did that same experimentwith birds there would be no helium in the very next exhalation.
Actually, birds have like a 4-cycle engine. It takes 2 completeinhalations and exhalation for a bird to process a given volumeof air. That first bunch are of air doesn't come back out againuntil the second exhalation.
And that's also associated with the presence of these air sacsand the unidirectional flow of air through the lungs. But becausethey have unidirectional flow of air through the lungs they aremuch more efficient at extracting oxygen. They can get more oxygenout of the same volume of air than a mammal.
And so there is an interesting experiment that was done a numberof years ago where a physiologist took a bell jar, one of thoseheavy duty glass chambers, and put a bird and a mouse in there.And he decreased the pressure so that it was the equivalent oflike, you know, 30,000 feet. And the mouse was comatose, it waslying there in the bell jar unconscious.
And bird was singing, having a great time. In fact, human beingstrucking up Mt. Everest breathing pure oxygen and not being ableto sleep and having horrible headaches, and being able to takeone step at a time watch ducks fly overhead quacking because theyhave extra energy to spare.
So birds' lungs are tremendously more efficient at extractingoxygen then are the lungs of any other kind of vertebrate animals,mammals being the one that are even close.
Birds also have twice a big of a heart.
Comparing, again, a fair comparison to the same body mass, birdand mammal, the bird has twice as big as heart as the mammals.And they also have a very large pectoral muscles. The musclesthat power the down streak of the flight. In a penguin, for example,30 percent of the body mass of the penguin is composed of justpectoral muscles, which is very unusual.
Now, so those are some adaptations of birds, how they mightcontribute some of the abilities of birds to fly, and some ofthem may not really hold water, but they're nevertheless true.Birds have a beak and no teeth, and very a reduced jaw apparatus.They have the old reptilian articular jaw joint. For example,it may not be because it saves weight but nevertheless it is true.
Birds' brains are also very different from the brains of mammals.Now, it's not usually a compliment to call somebody a bird brain.
And what is true is sometimes the behavior of birds makes themlook really profoundly stupid. When I was a graduate student atUCLA I was studying quail eggs and I had a bunch of quail. Andwe had what are called battery cages, like they have up in thepoultry unit. Which is a small cage. It's about that tall, aboutthat wide, that deep.
It has water at one end, food at the other end. And the floorthe slanted so the birds would sit in cages day after day, layeggs, eggs roll out and I was doing experiments on the eggs.
And I would go check those birds every morning. When I got towork at 8:00 o'clock, I'd open the door to the animal room wherethey were. And they would all do what quails do when they arefrightened and want to escape, is to leap straight up in the airand fly away.
Accept that the roof was only that far away. They would leapstraight up and bang their head and sort of look confused. Andthey did that day after day after week after month. They neverever learned that you can't leap up straight in the air when youare in a battery cage and fly away.
So birds don't exhibit a lot of ability to learn from theirexperience. And that's how as mammals and also as humans beingswe define intelligence. Although, we have some friends that areprobably not particularly good examples of this. The ability tolearn from experience is how we define intelligence.
And that certainly is highly adaptive features. But the birds'brain emphasizes not learning, but another means of developingcomplicated behavior, which is instinct. So the bird's brain emphasizesinstinct. And the mammal's brain emphasizes learning. And thoseare 2 different ways of achieving complicated behavior.
It's like if you have a simple calculator and your simple calculatormay have features that are hard wired into it, like calculatingsums and standard deviations and a lot of different functions,converting signs and cosigns. But that's not same thing as youhave in your home computer which you can execute a program. Youprogram it, like learning, you can do something brandnew, thatis built into the computer.
Your calculator is hard wired. That is instinct behavior. Birdshave some truly amazing sophisticated instinctive behavior builtinto their brains. Some of this is hard wired and the result ofnatural selection and connections between neurons that havedeveloped during the development of the bird.
Birds, for example, can go on these incredible migrations. It'snot usual, for example, in some species of ducks that nest upin the Arctic, high up above the Arctic circle, where in the fallan early storm can bring in a lot of snow and cold whether andkill everybody that's there. And so what happens when the adultbirds, the babies are still running around trying to find enoughfood to put on fat and grow up to adult body size. The adult birdsmay have fatten ed up and the adult birds will take off on theirmigration and leave all the kids behind, and every single adultleaves.
And two or three weeks later when all of the babies have puton enough food and enough fuel in the form of fat, they're readyto take off on flight. Then the baby birds all take off on migrationand they might travel in here in North America, they might traveldown the Mississippi fly way, stop at the gulf of the Mexico,wait for a good whether front, fly across the Yucatan, hit themainland, get to South America and fly into the exactly the samechunk of habitat as mom and dad are sitting in already.
And they make that entire trip with no adults, no bird makingthat trip has every done it before. It's all built-in, the instinctiveactive knowledge. What do you do when you get to the gulf of Mexico,how do you judge? They have the ability to use the stars as acompass. They have the ability to use the earth's magnetic field.They have ability to compensate for the movement of the sun'srays, so they can use the sun as a compass. They have an amazingcomplex instinctive behavior and that's all built in to theirbrain.
So it's a different solution. And they do stupid things becausethey don't learn from behavior, but they do incredibly sophisticatedand complicated things that it took human beings thousands ofyears to learn how to do. For us to go and sail around the worldor to go on a kind of lengthy boat trip that these birds do, youknow, it took the communication of massive amounts of learningover thousands and thousands of generations for human beings toget to the point where they could do that.
And birds are born knowing how to do that. So the birds brainis -- in fact, if you can compare the size of the brain to thebody mass of an animal, a birds' brain is just as big as the mammals'brain. They have just as big a brain comparison. That's much largerthan a reptile brain.
And remember I told you that mammals have evolved a neopallium,a new tremendously enlarged area of their brain. Birds do nothave neopallium. They have a structure called the "corpusstriatum."
And a lot of this very complicated behavior resides in populationsof cells in the corpus striatum. And just as the neopallium servesas an area of the brain for making sense of information obtainedfrom the sense of smell and the sense of hearing in mammals, thecorpus striatum serves, too, an area of integrating informationfrom the eyes.
Birds have almost no sense of smell. They're the only -- theonly species of birds that have any decent demonstrable senseof smell are the vultures. In fact, a group of birds that shouldn'thave any senses because they are always sticking their heads inrotting corpses.
But the corpus striatum serves as integration of informationfrom vision which is a reflection of the fact that birds are diurnal.And so the sense of sight is very important in a day-active birdor day-active animal.
But also because birds can fly. If you were moving around ina complex 3 dimensional environment where one mistake can meanthat you are dead, the sense is of smell or the sense of hearingisn't going to do it unless you are a bat. But we'll talk aboutthat more later.
What about some generalizations about the reproduction in birds.
I'm sorry did you have a question?
STUDENT: Well, parrots seem to have the ability to learn. Istheir brain bigger than other birds?
INSTRUCTOR: There are some species of birds, including parrots,that have some fairly good ability to learn as well as have alot of distinctive behavior. I don't think their brains are majorlylarger than a normal bird brain. I don't know that for sure. Butcertainly different types of birds do exhibit some degree of abilityto learn. Some of the cocks and owls can learn to do things aswell.
Reproduction or courtship in birds usually starts with somekind of a territorial display, frequently postures. That birdof paradise, that has a really amazing behavior. You know, whenI think I think it, I think that picture is upside down. In 16years, I never noticed that picture being upside down. And you'llsee a bird doing this in the "Life on Earth Videotape."This bird is hang upside down from a branch. And it's making anoise that sounds like a machine gun, and that's part of a courtshipdisplay.
So a lot of singing in birds is territorial display which meansit's directed at other males, not at females. Although, some singingis directed towards females. Lots of times male birds are -- femaleschoose their mates not on the experience of the male, but onthe basis of its territory. It's like choosing a boyfriend basedon the kind of car he drives.
Fertilization is internal usually by means of a cloacal kiss.Most species of birds have no intromittant organ. As I said, allbirds are ovoviparous and produce an amniotic egg with a calcareousshell.
Each species needs to incubate it's eggs for a particular lengthof time. That's called the incubation period. It's the time thatthe egg must be warmed up to a temperature close to body of theadult in order to complete the development. And those incubationperiods range from 11 days, being the shortest, up to, as alongas 60 days.
And the temperatures at which the incubation are kept are differentin different species. Some hold it 34, 35 degrees. Others as highis 38 degrees, but never as high as the adult bird.
Bird's body temperatures are higher than mammals' bodytemperatures. The bird's body temperature is about 40 to 44 Celsiuscompared to 37 degrees in most mammals. And egg temperatures are35 to 38.
So usually in the neighborhood of four or five degrees lowerthan the body temperature of the adult bird. But they do regulatethe temperature of the egg. There have been experiments wherethey change the air temperature and birds will actually sit ontheir eggs and prevent them from getting too hot.
Or if you put a bird in a place where the air temperature ishigh so the temperature will rise, then the adult wants to coolit down, it tries to keep the temperature from getting too high.It actively regulates the temperature rather. Incubation is 11to 60 days.
The general tend is to correlate with the body size of the adult,which correlates with egg size. Bigger eggs take longer to hatchthan little eggs. But there is a great deal of variability inthat relationship, and it correlates usually with the ecologyof the adults.
If a species is living in an environment where -- for example,the longest incubation periods in birds, that is 60 days long,occurs in species that nest on isolated islands out in the middleof the ocean where there are no predators around.
There is no selective advantage in getting -- hurrying up thedevelopment of the embryo or speeding up the development of thebaby after it hatches. So they have a very long and sort of leisurelydevelopment. So they have a long incubation period.
And so there are these kinds of evolutionary explanations forincubation periods. They tend have to a fairly fast incubationperiod in species where there is a short breeding season. Speciesthat migrate up to the Arctic to produce their young will generallyhave very short incubation periods. So if they take too long andthe snow falls then they lose the whole entire effort.
The young at birth are in a continuum, really. If we look atthe different species from the very primative condition whichis precocial along the continuum to altricial. Any single speciesis going to be at one point along this continuum.
But when we look at the 8,000 species of birds, we see manyof the most primative species are precocial. These are the onesthat are ready to get up and run around shortly after they hatch.Birds like ducks and chickens, for example, they are covered --their bodies are covered with down which is a type of featherthat serves for insulation.
They do a good job of regulating their own body temperature.They can locomote. Their eyes are open. They can find food. Theparents usually will stay with them and will keep them warm atnight when their own ability to thermo regulate might be exceeded.They will warn them of the approach of predators. Sometimes theywill defend them against predators. They may point out food tothem rather than let them just run around. So there is a lot ofparental care even though they are capable of locomotion.
On the other extreme, we have altricial, which is more highlyevolved, a more recently evolved condition that we find in themore recently evolved groups of birds, the song birds. And inthis case what you have is a baby that has no down when it's born.Its eyes may not be open so that it stays in the nest. And oneof the parents has to stay there and keep its body temperatureup to regulate its body temperature because it does not use itsmetabolism to do that.
Now, you might wonder why is that an advanced condition? Whenyou first think about it, it might seem like the wrong way togo. But what these birds do is they manage to have a very shortperiod of time from hatching to independence. From the time thatthe egg hatches to the time the babies are ready to go take careof themselves, that period of time is very short, because theyare not using energy on locomotion and thermo regulation. Theparents are the ones that are providing for those needs.
So the parents bring them food, keep them warm. The babies growfast and are out on their own and the parents can have anothergroup of young.
So what we frequently see is that in a species that producesaltricial young, the parents may produce two or three groups ofyoung during a single breeding season. Whereas the species thatare precocial, the parents may produce only one group of youngduring a breeding season. So the total number of babies that apair can produce may be increased by having altricial young.
So, obviously, when I talked a bit about parental care, it isusually extensive whether it is altricial or precocial. Again,there is some very interesting evolutionary variations on this.Not all of them fit into the total stereotype of being coveredwith down and locomoting versus naked and ugly, sitting in a nest.
There are some species where -- the young of hawks and owls which stay in the nest but are covered with down andable to thermo regulate. Their eyes are open. They are able ableto thermo regulate, but they can't get up and walk around becausethey're up in the nest way up a tree. Going for a walk could bevery hazardous. And they stay in the nest because it's reallydifficult to make a living as a hawk if you can't fly.
So I mean, if you are going to be precocial and looking foryour own food, you better be looking for food that doesn't runaway. What we see is insectivores producing precocial young thatgo out and look for food. And a predatory specie may produce youngthat have down so they can regulate their body temperature, whilemom and dad look for food. But they don't go out and look forit themselves.
Again, some interesting ecological correlations of these differencesbetween different species.