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    Birds were the second of three vertebrate groups to evolve flight (pterosaurs, birds, and bats).  In many ways, birds are the most advanced vertebrates because they have more derived characteristics than any other group.  They have a higher metabolism than even mammals.

      What makes a bird a bird?  This seems to be an easy question to answer since birds possess a number of features which distinguish them from other living vertebrates.  However, the answer becomes much more complicated when fossil groups are considered as well.  An enormous amount of evidence indicates that birds are descendents of dinosaurs and, not surprisingly, many “bird” features are also known in some dinosaurs as well.

1)     FEATHERS

FEATHERS FEATHERS
GULL FLYING      The presence of feathers is perhaps the easiest way to identify a bird: all birds have them and no other organism on earth possesses them.  Feathers serve a number of functions: they allow for flight in most birds, they serve as insulation which helps birds maintain their high body temperatures, and their coloration/arrangement may function in courtship or camouflage.  Ruby throated hummingbirds may have as few as 940 feathers while whistling swan had 25,216 (Van Tyne, 1976).  Feather color and arrangements can vary greatly even among closely related birds.
PARROT IBIS
IBIS
     Most birds arrange the bases of feathers in tracts with spaces in between.  When hatched, some birds possess no feathers (such as kingfishers and woodpeckers), others a small number, and others are coated in down. Young birds develop a juvenile plumage that they will molt to attain their adult plumage.  GEESE

The time of this molt is often about 3 months after hatching but it can be longer (four years in herring gulls).  In adult birds, feathers molt at least once a year.  Feathers are usually molted gradually but some birds (typically aquatic birds which can escape without flight) can molt their feathers all at once.  Some feathers form very fine filoplumes which are hairlike (Van Tyne, 1976).

       How did feathers evolve?  Anatomical, embryological and genetic evidence indicate that birds are more closely related to reptiles (especially crocodilians) than they are to other groups such as mammals.  Birds do possess scales on their legs and it seems that feathers evolved from modified scales.  The early development of feathers is similar to that of scales  (Van Tyne, 1976).

     A number of theropod dinosaurs are now known to have possessed feathers.

DINOSAUR WITH FEATHERS DINOSAUR WITH FEATHERS DINOSAUR WITH FEATHERS

   How did feathered flight evolve?  Did arboreal dinosaurs evolve feathers to help them glide from tree to tree and only later did these feathers cover the body for thermoregulation?  Did feathers evolve for thermoregulation and display in running dinosaurs which they developed the ability to fly?  Modern consensus rests with the cursorial (running) hypothesis rather than the arboreal hypothesis. 

     There have been fossils of individual feathers and until recently they were assumed to be the feathers of birds.  Because of a growing list of dinosaurs which were feathered, that assumption can no longer be made.  A number of finds have indicated that at least some dinosaurs had feathers, including Sinosauropteryx, Beipiaosaurus, Sinornithosaurus (all three had hairlike, seemingly unbranched structures), Microraptor, Caudipteryx, and Protarchaeopteryx  (which seem to be true avian feathers although Caudipteryx might be a primitive bird).   The feathers of Sinosauropteryx are simple, plumule structures which might represent an early stage in the evolution of feathers.  Microraptor is the smallest known dinosaur (about crow-sized) and its curved claws (similar to those of Archaeopteryx) perhaps indicate that it could live in trees. 

 

2) THE SKELETON

     The modern avian skeleton includes a number of traits which makes it unique among living vertebrates.   Many small bones have fused together to make larger composite bones: birds possess fused wrist bones and metacarpals (forming a carpometacarpus), fused ankle (tarsal) bones and metatarsals (forming a tarsometatarsus), a pygostyle composed of fused tail bones, a furcula (wishbone) composed of fused clavicles, and a synsacrum composed of fused pelvic bones and sacral vertebrae.  

BIRD SKELETON BIRD SKELETON
     Although these traits characterize all modern birds, some of them appeared later in bird evolution and are absent in many fossil birds.  For example, a number of early birds such as Confuciusornis lacked the fusion of wrist and ankle bones which are found in modern birds.
In the following image, many skull bones of this bird are fused together.BIRD SKELETON
CONFUSCIORNIS
The first birds possessed long, reptilian tails.  The fusion of the tail vertebrae to form the modern pygostyle occurred in a series of stages.  In modern birds, the number of free caudal vertebrae vary from 4-9.

tails of modern and fossil birds

       In the majority of birds, the two clavicles fuse to form a furcula (some exceptions in parrots, barbets, and toucans)  (Van Tyne, 1976).  A number of dinosaurs possessed furculas (“wishbones”) as did the early birds.

dinosaur furculas

The wings of birds are arms which have been modified for flight.bird arms

  The earliest birds possessed grasping hands with claws on the tips of their fingers.

archaeopteryx

It should be stressed that the homologies between bird and dinosaurian hands are enormous.  Archaeopteryx (the arm to the right in the drawings below) and Velociraptor arms (left) and hands virtually identical in structure including the same phalangeal count (the number of bones in each digit) and shape and the prominence of a semilunar wrist bone. 
velociraptor hand archaeopteryx hand
     Adult birds possess only two carpal (wrist) bones.  Other carpal bones exist in embryonic development but fuse with metacarpals to from the carpometacarpus.  Birds possess three fingers: the first usually possesses one phalanx (but two in some birds), the second finger possesses two phalanges (but three in some birds) and the third finger possesses one (Van Tyne, 1976).  Some modern birds still possess small claws on the tips of their digits.  One South American bird has clawed fingers as a hatchling.  Another South American bird possesses prominent claws as an adult:
claws on wing claws on wing

     Vestigial claws occur on the hands of a number of birds usually on the first digit but sometimes on the second as well, especially in young birds from Kiskadee flycatcher, white-necked crow, red winged blackbird, spur winged goose, jacanas, some plovers, and a few ducks (Van Tyne, 1976).

     The fossil bird Rahona also possessed a large retractable claw on its foot, which is especially interesting given that this trait was also present in the group of dinosaurs most closely related to birds.

 

rahona
In the avian foot, three toes face forward and one toe faces backwards (although some birds have modified this basic plan, such as owls).  Theropod dinosaurs also walked on three large toes.

Although modern birds lack teeth as an adaptation to lighten the skull, teeth were common in the earliest birds.  Although some fossil birds lost their teeth, the lineage which led to modern birds apparently retained their teeth until late in the Cretaceous.

sinornis

feet

     Some of the skeletal traits which are recognized as avian today were also found in the dinosaurs which were most closely related to them.  Modern birds have holes in many bones where air sacs invade them; this is called postcranial pneumatization.  Almost all tetanuran theropods have pneumatization of their cervical and anterior thoracic vertebrae just as in Archaeopteryx and modern birds.  Among the maniraptoran theropods, there is even more pneumatization.

In both birds and many dinosaurs, a series of small bones exists around the eye which form a sclerotic ring, although these bones fuse in birds.

eye ring

     Although modern birds possess a keeled sternum (a large projection on the breastbone seen in the image below) for the attachment of flight muscles, this trait was absent in the earliest birds.

bird sternum

The number of ribs varies in modern birds.  In modern birds, the fibula is reduced and usually does not extend to the ankle (except in some penguins, ospreys, and eagle owls.  The relative lengths of the upper and lower jaws vary.   (Van Tyne, 1976).  There can be considerable variation in the length and shape of the jaws, even within a family of birds.
beak differences beak differences

 

3) OTHER ANATOMICAL FEATURES

   Birds are “warm-blooded” or endothermic.  Endothermy has evolved separately about a number of times in animals: it has been observed in some sharks, bony fish, moths, dragonflies, beetles, and bees.  The ability to maintain a constant body temperature allows birds to be active at cooler parts of the day and night and to inhabit colder parts of the world.  Flight is such an energy-intensive activity that birds require the high metabolic rate which enables flight, regardless of the environmental conditions.  In flight, the oxygen requirements of birds can increase more than 20-fold.  Passerines have been known to reach 55 mph in flight, ducks can reach 50-60 mph, and peregrine falcons can reach 175 mph.  Hummingbirds may beat their wings up to 52 beats per second.  There is a good deal of evidence that many dinosaurs (such as those which are closely related to birds) also had some degree of endothermy (given their feathers, ability to live near both the north and south poles, their upright stance, their large brains, predator to prey ratios, the microscopic structure of their bones, and other features).

    An advanced nervous system is also a characteristic of birds: birds possess a large brain and acute senses.  Although the cerebrum is large, its increased size is primarily due to an expansion of the basal nuclei (such as the striatum) and not the cerebral cortex (the region which is expanded in mammals).  In many birds, the brain can be rotated on an axis: 15 degrees in comoronts, 47 degrees in owls, 117 degrees in a woodcock.  The midbrain is prominent in bird brains, particularly the optic lobes.  The cerebellum can be small in flightless birds but large in flying birds.

     Bird tongues can be modified as a probe and for other functions.  In some families of birds there is a permanent dilation of the esophagus to form a crop, in others it can form temporarily.  The stomach is divided into a preventriculus and a gizzard, although in some birds the gizzard is rudimentary.  Food can be ground in the gizzard.  This ability allows wild turkeys, wood ducks, and mallards can swallow acorns and nuts (Van Tyne, 1976).

     In birds, the gonads grow during breeding season--the testes may increase up to several thousand times their normal size.  A cloacal penis exists in some families of birds.  Most birds only possess a left ovary but in some hawks the right functions as well.  The small right ovary may be retained as an ovary, a testis, or even an ambisexual organ which can cause sex reversal if the left ovary is destroyed. 

     Fertilization is internal in birds and must occur before shell and albumin secreted around embryo (Van Tyne, 1976).  Developing eggs are visible inside this chicken.

eggs inside chicken eggs inside chicken

     Birds possess a renal portal system that can be used at will because of a renal valve.  Embryonic birds possess contractile lymphatic hearts which may be retained into adulthood (Van Tyne, 1976).

4) BEHAVIOR

     Many birds utilize a variety of complex behaviors during courtship, which include a great diversity of vocalizations.  In addition to vocalizations, birds can make sounds by stamping their feet, snapping or rattling their jaws, or pounding their bills on an environmental object.  A few birds such as bitterns and sage grouse can pump air into their distendible esophagus and produce sound upon its forceful release.  Although the syrinx is the primary source of sound, there are other areas such as the mouth and coils of the trachea which can modify sounds (Van Tyne, 1976).

     Bird young typically receive parental care.  Fossils indicate that at least two types of theropod dinosaurs brooded over their nests in a bird-like pattern.

oviraptor

     In migration, birds use stored adipose as an energy source for extended flights.  In migrating, birds can go back and forth between definite summer and winter sites.  While some North American birds migrate only a few miles, others can migrate distance of more than 7,000 miles (the latter in the cases of Arctic shorebirds and the Arctic tern) (Van Tyne, 1976).

migrating birds migrating birds

     Some birds can enter into torpor at night. 

 

     The most primitive birds alive today are the flightless birds (such as ostriches and rheas) and a group of small South American birds which seem to be related to them (the tinamous pictured below).

tinamou tinamou