December 2008



Nursing fawn

Last month we learned that although birds and mammals have unique differences, they are closely related. Birds have much more in common with each other than mammals have with each other. In weight only a 1 thousand fold difference separates the smallest hummingbird from the largest condor whereas a 68 million fold difference separates the smallest shrew from the largest whale. And all birds fly; even penguins flap their wings to swim. But mammals have many more specialized modes of locomotion; they fly or swim, they walk and gallop, they climb and swing through treetops, and they burrow and jump.

Since all birds fly, they cannot have the diversity of structure that mammals have. To explain how flight constrains the structure of birds we now explore the extreme specializations of birds for powered flight. We do this by comparing bird and mammal anatomy, physiology, and development.


The evolutionarily adaptations for flight are the same in all birds and so it is hard to find anatomical differences that allow us to separate and classify them. Remember the point from last month that a bird is a bird is a bird. One result is that classification of birds is fearsomely complex and contentious.

David Sibley (The Sibley Guide to Bird Life and Behavior) succinctly summarizes bird specialization as "life without forelegs." Bird forelegs are wings used to fly and this has had many consequences. To paraphrase Sibley, birds have many more neck vertebrae than mammals, so they can use their beak to reach objects and groom. What mammals do with their forelimbs to feed and groom, different bird species accomplish with a tremendous variation in their bills and feet.

If you decided last month that a unique feature of birds was wings for flight, you were wrong! If you answered feathers, you were correct!

Flying birds have evolved a light weight and compact body build. Although feathers make up 15-20 % of a bird's total weight, they make up nearly 40% of a bird's apparent volume. Amazingly, a bird's feathers weigh twice as much as all of its bones, many of which are hollow. A bird's body cavity has many air sacs that even extend into some hollow bones. Because they have a high volume of feathers and have air sacs, a bird looks big and everyone far overestimates its weight. For the same body length, width, and thickness, a bird weighs four times less than a mammal! In addition, a bird's skull, with its small mandibles and hollow beak, is very light but a mammal's skull with its massive jaws and solid teeth is very heavy.

Despite the light weight of the bird skeleton, it is specialized in many ways to withstand the mechanical stresses of flight. Compared to a mammal's skeleton, a bird's skeleton is reinforced in two ways. First, birds have many fewer bones than a mammal because some are lost and others are fused together. This provides less flexibility but greater strength. For example our arm has 29 bones while a bird's wing has only 11 separate bones. And our adult skull has 20 bones while a bird's adult skull has only 11 bones. A bird's breast bones, ribs, and vertebrae are fused into a compact pectoral girdle that anchors the muscles of flight. Its lumbar vertebrae and pelvic bones are also fused. A second means of reinforcement of the bird skeleton is a honeycomb of struts inside its hollow bones.


Flight is very efficient but very expensive metabolically. Flying birds have 5-20 times their resting metabolic rate whereas running mammals only have 2-4 times their resting metabolic rate. To "pay" the high cost of flight, birds have incredibly efficient and very high rates of metabolism, respiration, circulation, and digestion. These allow both fast flight and long distance migration.

High rates of metabolism in birds are associated with high body temperatures and efficiency of circulation and respiration. Flying birds have body temperatures 4°-5° C higher than running mammals. A bird of the same size as a mammal has twice the heart size of a mammal. Birds also have much more efficient lungs than the tidal flow lungs of mammals. Air passes through bird lungs in one direction into posterior air sacs and then back through the lungs by a different route. As a result, both of two passes of air in each respiratory cycle and of countercurrent flow, the oxygen extraction is very high. This efficiency of respiration and circulation enables birds to fly over the Himalayans at the altitude of Mount Everest where the air has such a low concentration of oxygen that no mammal can survive. Interestingly the first birds and the largest flying reptiles (Pterosaurs aka Pterodactyls) started to evolve flight from the Jurassic to the Cretaceous when some data show that atmospheric oxygen was much lower than today.

Now we can answer the question from October about why birds migrate south in winter but mammals do not. The simplest answer is that birds can fly and so can travel long distances quickly and efficiently. To stay warm in winter they would have to increase the depth and insulation of their feather coat to such an extent that they could not fly efficiently, if at all. To stay warm in winter, northern mammals develop a thicker and better insulating winter coat but this does not slow their ability to walk or run. Larger mammals, like deer and moose, can move long distances to protected pine and spruce forest in the coldest times. Many medium and small mammals burrow beneath the insulating snow and maintain winter runways. Others hibernate, including insect-eating bats.


The fast and efficient digestion of birds also complements their high metabolic rate. Fast and efficient digestion is an adaptation related to the great energy need for flight. A corollary is that birds cannot eat foods, especially plants, that are not easily digested.

Mammals have lower metabolic demands and do not need fast and efficient digestion. Perhaps this explains why so many kinds of mammalian herbivores have evolved. Our largest land mammal, the African elephant, is 375 times the weight of the largest flying bird, the great bustard. Mammalian herbivores have very long intestines and the largest species also have specialized compartments in their digestive tracts. These compartments, like the rumen of cows and the caecum of rabbits, have symbiotic bacteria and protozoa that have the enzymes necessary even for the digestion of plants with high cellulose concentrations. They must chew their food for long times and some even regurgitate it to chew more after enzymes and bacteria have been added from specialized compartments. Perhaps you have heard of cows chewing their cud.

Since they do not have teeth, birds cannot chew their food. Thus they either swallow very small pieces, like songbirds that eat insects and seeds, or can open their mouths very wide and have very distensible throats to swallow large prey, like raptors and herons that eat mammals or whole fish. To grind swallowed food into smaller pieces birds have a muscular gizzard that is lined with tough keratin and they eat sand and gravel to help grind food in their gizzard.

The need for a flying bird to minimize weight and have a high metabolic rate may explain why they have short but very efficient intestines. All birds need to completely digest a meal quickly to get energy fast enough, usually in an hour or less, to fuel flight. In contrast a mammal of similar size takes most of a day from ingestion to egestion. This difference relates to why birds are not plant eaters.

Herbivory requires a long time to process food with a long specialized digestive tract and is not very efficient.

Very few birds are omnivores that eat both plant and animal material. The plant material that birds do eat is low in carbohydrate, like cellulose, and high in sugar or fat and protein, like seeds and fruit. Birds like pheasants and quail have numerous small outpockets of their intestine that slow passage of seeds and so increase the efficiency of digestion.


The way food is handled initially is reflected in many different teeth types in mammals and a few different beak shapes in birds.

omnivorous dentition of a coyote

Let us start with our own mammal teeth. Use your finger to feel, or a mirror to look at, or "play-doh" or chewing gum to make an impression. Determine the size, shape, and sharpness of your incisors, canine, premolars, and molar teeth. Which teeth nibble or bite? Which ones might pierce or stab? Which might slice or shear and which definitely chew and sometimes grind? Eat some different foods to help decide what each kind of tooth does. Do your teeth do different things? If so does this allow you to eat a diet of many foods? If yes, we conclude that you are a generalist feeder, an omnivore. In generalist feeders like humans, and to a lesser extent coyotes and dogs, the teeth are of all types and all are of intermediate size and complexity.

We often contrast the advantages (+) and disadvantages (-) of generalists and of specialists. A generalist we call a jack-of-all-trades (+) but master of none (-). The plusses and minuses of a specialist are the opposite. A specialist is great at one thing (+) but poor at everything else (-).

Among carnivorous mammals, bobcats are the most specialized feeders. They have tiny front incisors, long sharp canines, and shearing premolars and molars. If you watch a cat chew it uses one side of its mouth, and the upper and lower shearing teeth are offset so they work like scissors.



Among herbivorous mammals, rabbits and beavers are the most specialized feeders. They have a pair of large, self-sharpening incisors, no canines or premolars, and flat-topped molars with complex grinding surfaces. Their teeth are continuously worn down as they bite and chew abrasive plant food that often has silica glass in it and tough woody lignin. This is the reason that their incisors and molars grow throughout their lives. If you watch them chew their food, you will see that their jaws move back and forth, side to side, and up and down as they slowly grind up plant food.

Since we can see a bird's beak, we can easily hypothesize what it might eat. We can watch birds catch, handle, and swallow different foods to test our hypotheses based on shape of the bill. The sharp thin bill of herons and egrets spear fish. The hooked bills of hawks and owls grab and rip apart prey. And the short conical bills of sparrows crack seeds.




It could be argued that shelled eggs in a nest and parents that feed babies are two more adaptations for flight. If birds carried multiple developing young internally this would certainly compromise flight efficiency. By laying eggs one at a time the weight of carrying developing young is minimized. For predatory birds with unpredictable food supplies, an additional advantage of laying eggs one at a time and incubating them from the time of laying is that babies hatch a day apart. The oldest hatchling, the first to hatch, is bigger, stronger, louder and gets the most food. This is a fail-safe mechanism to ensure that at least one young survives. If food is plentiful even the youngest hatchling will survive. A constraint on birds' reproductive mode is that food must be available for the adults to find easily to feed growing young in a nest. So, unlike mammals, birds reproduce only when and where food is abundant, usually in spring (see October column on the advantage of migration).

bird embryo in shelled egg


Birds are unique in that they have a hard-shelled egg with a special sac to store uric acid wastes and have yolk to provide nutrition to the developing embryo. After hatching, the parents must search for items to feed the babies.


swift feeding newly hatched baby bird

a specialist carnivore (bobcat) has tiny incisors, huge canines, and shearing molars






Mammals are unique because their young develop internally and are nourished by a placenta. Their locomotion is not seriously compromised when females carry multiple enlarging young in a uterus with placental exchange of nutrients and wastes.


mammal fetus with fetal & maternal parts to the placenta


Another unique feature of mammals, is that after birth young suckle to get milk high in fat and protein. An advantage is that the mother can eat many different foods and store fat so that she can provide milk even if the food supply fluctuates. So, unlike bird reproduction, mammal reproduction is not necessarily tied to a flush of plant and insect production in spring.




Spring is here,
The bird is on the wing.
Don't be absurd,
Everyone knows the wing is on the bird!


a. A bird's feathers weigh about twice as much as its entire skeleton.

b. A mammal's lungs are different than a bird's lungs but just as efficient in extracting oxygen from breathed air.

c. The bird's gizzard performs some of the functions of mammal's teeth.

d. Based on human teeth types we may hypothesize that we are generalist feeders, i.e. omnivores

e. Like mammals, some birds are specialist herbivores.