Ancestry, Evolution, and Decrease of Birds

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Despite the astronomical numbers of birds that have lived and died in the passing of millions of years, the fossil record is meager when compared with other vertebrate groups. Bird bones are usually small and fragile and are therefore less likely to be preserved than the massive bones of mammals and some reptiles. Most bird bones that have been preserved as fossils are parts of legs, wings, or vertebrae, and occasionally a skull or sternum is recovered. Whole skeletons are unusual. However, in the past decade many new bird fossils, both single elements and whole skeletons, have come to light, increasing considerably the understanding of avian evolution.

Ancestry Undeniably, birds arose from reptilian stock. In the first place, modern birds and reptiles show numerous physical features in common. For example, their skulls bear one rather than two occipital condyles for articulation with the vertebral column; their lower mandibles articulate with movable quadrate bones as in lizards and snakes instead of being hinged directly on the skull; and their ears have a single bone, the columella, for conducting vibrations from the tympanum across the middle ear to the internal ear. In the second place, there is the highly important evidence of reptilian ancestry from Archaeopteryx lithographica, an ancient winged creature whose fossil remains were found at six different times in lithographic limestone near Solnhofen, Bavaria.

The first discovery, in 1860, comprised the imprint of an undoubted feather. The second, the next year, consisted of an incomplete skeleton (now in the British Museum) showing particularly forearm and leg bones; a long tail with many vertebrae and the impressions of feathers, some attached to the forearms; and a pair each to most of the tail vertebrae. The third discovery, in 1877, was a skeleton (now in a Berlin museum), virtually complete except for the lower mandible, the right foot, and some of the cervical vertebrae. The fourth discovery, in 1958, was another skeleton (now at the University of Erlangen), much less complete than the two found 80 years or so earlier. The other two specimens were discovered in 1970 and 1973 in museums; both were recovered earlier, but they were misidentified as a pterosaur and a small dinosaur, Compsognathus. A little larger than the Common Pigeon, Archaeopteryx lived in the Late Jurassic Period (see Table 8), some 140 million years ago, at a time when reptiles were dominant vertebrate animals. Without the tell-tale feathers in the fossil remains, the discoverers might have identified it as just another reptile from that remote age. But now that numerous authorities (e.g., Heilmann, 1927; de Beer, 1954) have scrutinized and evaluated the 1861 and 1877 specimens and studied the 1861 specimen by direct and indirect lighting and by ultra-violet and X-rays, they consider Archaeopteryx more bird than reptile with nonetheless remarkable and— from an evolutionary viewpoint—significant features intermediate between the two. The skull has such reptilian features as an overall heavier structure, large fossae in the facial region, 351

352 / Ancestry, Evolution, and Decrease of Birds Table 8

Geologic Time Scale

Era

Period QUATERNARY

Epoch

Million years before present

Recent

0.01

Pleistocene Pliocene

CENOZOIC

Miocene

(AGE OF BIRDS AND MAMMALS)

TERTIARY

Oligocene Eocene Paleocene Late

CRETACEOUS

Late

MESOZOIC (AGE OF REPTILES)

Early

JURASSIC

Middle Early

TRIASSIC

1.5-3.5 •7 26 37-38 53-54 65 100 135 155 170 180-190 230

From Feduccia, 1980, in The Age of Birds, Harvard University Press, Cambridge, Massachusetts.

no bill, and pointed teeth in sockets. Also, the brain-case shows that the cerebral hemispheres w e r e elongated, the optic lobes dorsal in position, and the cerebellum small and not extended forward to overlap the posterior parts of the cerebral hemispheres. The orbit, however, is relatively large and contains a sclerotic ring. The vertebral column, comprised of 10 cervical, 12 thoracic and lumbar, 6 sacral, and 20 caudal vertebrae, is distinctly reptilian, with little if any fusions of vertebrae. The vertebral centra are biconcave or amphicoelous—simpler than the saddleshaped or heterocoelous type in modern birds. The caudal vertebrae are elongated and free, with no evidence of their fusing terminally to form a pygos-

tyle. All the ribs are unjointed, lack uncinate processes, and do not reach the sternum. The sternum itself is short, broad, and unkeeled, while posterior and lateral to it are about 12 pairs of dermal ribs or gastralia which are present in the ventral abdominal wall of some reptiles. The pectoral girdle shows, besides a scapula and coracoid, a clavicle fusing with its fellow of the opposite side to form a distinctly avian feature, the furcula or wishbone. The forelimb is as long as the hindlimb. Its humerus is longer than the ulna, and the ulna is a little longer and stouter than the radius, but none of the t h r e e bones is very strongly developed. In the wrist are five carpals, two ( the radiale and

Ancestry I 353 ulnare) proximal and three distal. The hand has three metacarpals. Whereas modern birds have only the radiale and ulnare separate and the other carpals and the metacarpals fused as one bone, the carpometacarpus, Archaeopteryx shows all the carpals and metacarpals separate with the possible exception of the outermost distal carpal being fused with the outermost metacarpal. Articulating with the metacarpals are three separate digits or fingers (the middle one longest) terminating in conspicuously long, curved, sharply pointed claws. Each digit is comprised of more phalanges than its homologue in modern birds. The pelvic girdle is connected to, rather than fused with, the sacral vertebrae, and its three bones—the ilium, ischium, and pubis—are separate and individually identifiable instead of being fused into one bone. The pubis is distinctly avian by being long, slender, and directed posteriorly. At the distal end it fuses with its fellow of the opposite side to form a symphysis. In the hindlimb the fibula is a separate bone alongside the tibia and extends all the way from the femur to the heel. The fibula and tibia are longer than the femur. The second, third, and fourth metatarsals remain separate and parallel for most of their length distally; only at their proximal ends are they fused with one another and certain tarsals to form the tarsometatarsus. As in the Common Pigeon and most other modern birds capable of perching, there are four digits or toes, the first (hallux) projecting backward. The fourth or outermost digit has four phalanges instead of five. However strikingly birdlike some of the skeletal features of Archaeopteryx may be, none is uniquely avian as are the feathers. Careful studies of their impressions in lithographic limestone show them to be in every way typical of modern flying birds. The forearm appears to have borne true primaries (as many as eight, according to Savile, 1957) and ten secondaries, each with overlapping coverts. Although the tail is more reptilian than avian in length and number of vertebrae, it supports rectrices— one pair with coverts attached to each 15 caudal vertebrae beginning with the sixth. Besides the feathers of the forearms and tail are the impressions of numerous other feathers which, from their positions and size, clearly suggest that the body, neck, and legs had a feather covering.

Nobody knows precisely what Archaeopteryx looked like in life, how it was shaped, and whether or not it was brightly or somberly colored. Numerous artists, however, have reconstructed it partly from anatomical evidence and the rest from supposition. See, for example, some very meticulous attempts in the paintings by Heilmann in the frontispiece of his book (1927), by R.T. Peterson (in Fisher and Peterson, 1964), and by Rudolf Freund (see Fig. 56). While Archaeopteryx shows beyond a doubt that birds arose from reptilian stock (Thulborn and Hamley, 1982), it is only to a limited extent helpful in providing clues to the ancestry of birds. Existing contemporaneously with Archaeopteryx were two groups of reptiles—the pterosaurs (winged reptiles) and smaller bipedal (two-footed) dinosaurs—which shared many of the skeletal features with birds. The pterosaurs, while equipped with pneumatic bones as are modern birds, depended on wings with membranes and with bones of different number and proportions. The bipedal dinosaurs had long hindlimbs on which they could run while keeping their forelimbs off the ground. Their pectoral girdles, however, lacked certain features, including a furcula, that modern birds possess. Obviously, these two groups of reptiles were no more ancestral to modern birds than Archaeopteryx. To find the ancestor common to birds (including Archaeopteryx), pterosaurs, and dinosaurs, one must search farther back in time for a more generalized form of reptile. Speculation, based on current researches, points to the Pseudosuchia, a small group among the extinct thecodont (socket-toothed) reptiles, which existed in the Old World during the early to middle Triassic Period some 200 million years ago. The pseudosuchians were bipedal reptiles with hindlimbs that were longer than the forelimbs and used for walking, running, and jumping. Their skulls had many of the basic features found in birds. In one form, Euparkeria capensis from Africa, the skull had all the bony elements present in Archaeopteryx, although it was more heavily constructed with a higher profile and stronger jaws. The orbits and brain-case were smaller. In another form, Ornithosuchus woodwardi from Scotland, the skull was more lightly constructed and thus more

354 / Ancestry, Evolution, and Decrease of Birds

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Evolution I 355 closely resembled that of Archaeopteryx. The feet showed a tendency toward reduction in the fourth and fifth toes. As in other pseudosuchians, Ornithosuchus supported a paired row of scale-like epidermal plates on the dorsal surface of the back and tail. Besides Euparkeria and Ornithosuchus there were many other similarly generalized forms of pseudosuchians existing at the time. It could have been from any one of these forms—or even from an older and still more generalized form which lived in the preceding Palaeozoic Era—that birds branched off, but more fossil evidence must come to light before there can be any certainty. The stock from which modern birds evolved has yet to be discovered.

Evolution Early Evolution If the numbers of early bipedal pseudosuchians were as vast as commonly believed, one can assume that they were subject to considerable competition for food as well as living space. Being carnivores by virtue of their socketed teeth, they were forced to seek small animals in every possible situation. At least one form of pseudosuchian and possibly more used their sharp claws to advantage in climbing trees where, through the continuing process of adaptive radiation, they developed physical attributes and techniques that enabled them to forage successfully. From an arboreal habit thus acquired, they proceeded (as suggested by Heilmann, 1927) to attain ever greater proficiency by jumping from branch to branch. This required the simultaneous development of more muscular power in their hindlimbs for leaping and of opposable first toes (halluces) for grasping. The next probable step in development was the extension of jumping movements to gliding from higher to more distant lower branches and eventually from high in one tree down to the lower part of another. The evolutionary consequences of these feats were the gradual acquisition and elongation of scales from the trailing edge of the forelimbs and (for balance as well as sailing surface) the expansion and lateral projection of the scales along the tail. In

due course the scales became lengthened and elaborated to form contour feathers. Archaeopteryx well represents that stage in the evolution toward flight where the subject became adapted to an arboreal life and capable of extensive gliding. Its feet were suited to grasping and perching; its forelimbs, while retaining some grasping ability as evidenced by the three-clawed fingers of the hands, were elongated and more suited to supporting the body in the air. The great extent to which Archaeopteryx depended on gliding is reflected by a number of anatomical features—for example, the lightening of the skull; the increase in size of the orbits to accommodate larger eyes for greater distance perception and visual acuity; and the larger brain-case to accommodate the enlargement of those brain centers that control and coordinate the more complicated neuromuscular mechanisms for gliding. With true contour feathers already formed in Archaeopteryx, and with no evidence in the few fossil remains of early reptiles that even remotely suggest how they formed, the origin of feathers is a fruitful field for speculation (see a brilliant paper on this subject by Parkes, 1966). The general consensus is that feathers developed first on the forelimbs and tail of an early arboreal reptile by the lengthening, broadening, and overlapping of those scales that would increase sailing surface and at the same time be lighter without sacrificing strength and durability. Since the trend toward lightness is of selective advantage to any airborne animal, it was quite natural for the modifying process to involve the remaining scales on the forelimbs and tail and eventually all the scaly covering of the body, neck, and head. A secondary advantage to feathers was no doubt to streamline the body by smoothing over angular surfaces and folds from head to tail. All of the feathers were contour feathers and developed initially as an adaptation to gliding. All other kinds of feathers found in modern birds—down feathers (both teleoptilian and neossoptilian) and filoplumes—presumably evolved from the contour feathers of the type in Archaeopteryx. After Archaeopteryx comes a wide gap in the fossil record until the Cretaceous Period some 20 million years later. In this long interval the evolution of birds must have accelerated, for by the Cretaceous, if one may judge by the several species

356 / Ancestry, Evolution, and Decrease of Birds discovered, birds were not only well established and globally distributed but had taken divergent paths of development, many becoming aquatic and many others remaining terrestrial. Among the species were the first known representatives of Charadriiformes and representatives of other orders, now completely extinct, that are known by fossil material from sedimentary rocks laid down by the great inland sea that occupied the west-central portion of North America for some 40 million years. Nearly complete skeletons have been found for some of these birds, notably Ichthyornis victor and Hesperornis regalis. Ichthyornis, a superficially gull-like bird about the size of a Common Pigeon, was obviously capable of sustained flight, for it had well-developed wings with a carpometacarpus fully formed, strong pectoral girdles amply supported by a furcula, and a keeled sternum for the attachment of large muscles to control the wing strokes. Hesperornis, a huge bird five to six feet long (1.52-1.83 m) was highly specialized for diving—perhaps the equal of loons in diving ability—and totally flightless. Far back on its elongated body the hindlimbs, with their paddlelike feet, projected at right angles to the body axis. Probably it could maneuver on land only to the extent of reaching its nest. The wings were vestigial, with no bones distal to a slender humerus; the pectoral girdles were not only weak but the clavicles failed to meet at the midline; and the sternum was flat and unkeeled. Whatever powers of flight the bird once had were long since lost. The skeletal structure of Ichthyornis and Hesperornis, though widely divergent for purposes correlated with locomotion, shared certain similarities such as a typical avian tarsometatarsus, a small skull with bones well fused, and the reduction in the number, and the fusing of, the terminal caudal vertebrae to form a pygostyle. In Hesperornis and Ichthyornis both mandibles retained reptilian teeth, fewer in the upper mandible than the lower. Hesperornis had heterocoelous vertebrae characteristic of modern birds, but Ichthyornis had amphicoelous (bi-concave) vertebrae that were peculiarly enjoined as they are in fish. This accounts for the name Ichthyornis—fish bird. Ichthyornis was undoubtedly able to fly in the manner of modern birds and, according to the fossil record of other Cretaceous birds, was not alone in

having achieved this ability. Indeed, except for the skull, the skeleton of Ichthyornis was that of a modern bird. Presumably, during the merging of the Late Jurassic and Early Cretaceous Periods, the feat of gliding, as exemplified by Archaeopteryx, was rather quickly extended for increasingly longer distances by flapping the forelimbs, thus sustaining the weight of the body against gravity. This gave the performer such selective advantages as capturing insects on the wing and more readily escaping from enemies and unfavorable aspects of the environment. In the development and achievement of flight the long tail that was advantageous only in sailing was foreshortened and its feathers directed posteriorly and spread fanlike to provide a mechanism for steering and braking; the eyes were enlarged further to give still greater visual acuity; and the teeth played a steadily decreasing role and ultimately disappeared, allowing the mandibles to be lighter in weight. The attainment of flight was unquestionably accompanied by a heightening of body metabolism through modifications in the organ systems. Cretaceous birds such as Ichthyornis and Hesperornis must have been homeothermous (warm-blooded) but how early in their development birds acquired a thermoregulatory mechanism is problematical. Some authorities contend that Archaeopteryx was at least somewhat homeothermous, as were its pseudosuchian ancestors and the pterosaurs, and that the feather covering of Archaeopteryx developed to hold its body heat as it took to an arboreal life in which there was more exposure to lower air temperature. The pterosaurs were able to fly successfully without any such special covering. It is more likely that the feather covering developed to facilitate gliding and later flight and that, as warmbloodedness increased with the attainment of flight, the feather covering eventually assumed an insulating function. Later Evolution Athough the fossil record of the Cretaceous is poor, advances in the avifauna must have been great, for with the coming of the Tertiary Period toothed birds had disappeared and a host of new forms began to emerge. Among them are the earliest

Loss of Flight known representatives of a great n u m b e r of modern families or orders that include the penguins (Spheniscidae), rheas (Rheidae), loons (Gaviidae), tropicbirds (Phaethontidae), anhingas (Anhingidae), cormorants (Phalacrocoracidae), herons (Ardeidae), ducks (Anatidae), vultures (Cathartidae), hawks (Accipitridae), grouse (Tetraonidae), cranes (Gruidae), rails (Rallidae), sandpipers (Scolopacidae), auks (Alcidae), cuckoos (Cuculidae), owls (Strigidae), swifts (Apodidae), trogons (Trogonidae), and the first few forms of Passeriformes. By early Tertiary time the proliferation of birds was occurring at a very rapid rate. Gone were the dinosaurs, pterosaurs, and other early reptiles that had dominated the Cretaceous, leaving habitats more than ever available to birds. Besides the proliferation of many families of modern birds and many forms obviously ancestral to modern species, there also appeared several gigantic flightless birds that seem to have replaced the giant reptiles of the Cretaceous. They thrived until the emergence of the giant mammals, and then died out, leaving no known descendants. A notable example was Diatryma from the Paleocene and Eocene of North America and E u r o p e . Specialized for terrestrial life, with massive, powerful legs, a huge head with an enormous bill, and small degenerate wings, it stood nearly seven feet tall (2.13 m). The fossil deposits of the Oligocene in the mid-Tertiary yield, among other forms, the first known grebes (Podicipediformes), albatrosses (Diomedeidae), shearwaters (Procellariidae), storks (Ciconiidae), turkeys (Meleagridinae), limpkins (Aramidae), plovers (Charadriidae), stilts (Recurvirostridae), gulls (Laridae), pigeons (Columbidae), parrots (Psittacidae), and kingfishers (Alcedinidae), together with a few more passerine birds. In the Miocene and Pliocene deposits, toward the close of the Tertiary, the fossil remains of birds are much more numerous and varied. While the majority represent forms already known, others are the first recorded evidences of ostriches (Struthionidae), tinamous (Tinamidae), falcons (Falconidae), oystercatchers (Haematopodidae), goatsuckers (Caprimulgidae), and several families of passerine birds. From the Pleistocene Epoch or Ice Age come the first records of the cassowaries (Casuariidae), emus (Dromiceidae), elephant birds (Aepyornithidae),

I 357

moas (Dinornithidae), kiwis (Apterygidae), ospreys (Pandioninae), jacanas (Jacanidae), phalaropes (Phalaropodinae), skuas (Stercorariidae), barn-owls (Tytonidae), hummingbirds (Trochilidae), motmots (Momotidae), and many passerine species. Undoubtedly, these birds evolved much earlier, in the Tertiary, since most all species living today and those recently extinct are believed to have been in existence at the beginning of the Pleistocene and well established in the Pleistocene. This may not have b e e n the case with some of the "higher'' passerines, which are thought to have acquired their species distinctions as a result of the Pleistocene. For example, see Mengel (1964) or a partial summation of his paper in this book, page 115. In its abundance and variety of forms the fossil record of the Pleistocene is the richest of all the epochs. Besides 732 living species, represented by fossils, there are 270 extinct species. One, a condorlike bird, Teratornis incredibilis, with an estimated wingspread of 16-17 feet (4.88-5.18 m), may have b e e n the largest bird ever to fly. Several Pleistocene deposits have yielded many hundreds of bird bones, but the most productive of any in the world are at Rancho La Brea near the center of Los Angeles, California, where during the Ice Age countless n u m b e r s of birds became entrapped in asphalt and their bones preserved in beds of tar.

Loss of Flight In the long history of birds various forms have been characteristically flightless—i.e., unable to fly because either they lacked wings altogether or they did not have wings large enough and powerful enough for sustained locomotion in the air. Most students of paleontology agree that this condition developed secondarily in all instances from stock at one time capable of flight. All birds, whether they can fly or not, are recognizable structurally as such because they evolved essentially as flying creatures. Hesperornis, Diatryma, and a dozen or more other genera of birds, long since extinct, were flightless. Among modern birds and some only recently extinct are a considerable number of flightless birds, two well-known groups being the ratites and the penguins.

358 / Ancestry, Evolution, and Decrease of Birds The ratites, so called from their unkeeled and consequently raftlike sternum, comprise the ostriches (Africa), rheas (South America), emus (Australia), cassowaries (New Guinea and Australia), kiwis (New Zealand), and the recently extinct moas (New Zealand) and elephant birds (Madagascar). All are terrestrial, with strong legs and feet, and all, except the kiwis, are large and heavy bodied. Many ornithologists feel confident that the birds arose independently in their respectively isolated areas and became similar through convergence; but a few (eg., Bock, 1963) feel differently, believing that the birds arose together and then dispersed to remote areas, perhaps before losing their ability to fly. (See Feduccia, 1980, for a complete discussion of the evolution of the flightless state in birds.) Regardless of how they arose, one may postulate that their ancestors took to living in treeless country w h e r e they became grazers. The cursorial habit of walking about for food and running to escape enemies became increasingly adequate and flight decreasingly essential; hence, their legs and feet grew stronger while their wings and keeled sternum degenerated. In some instances birds, isolated on islands and free from predation by carnivores, grew to gigantic size. In New Zealand one of the 12 species of moas, Dinornis maximus, stood 12 feet tall (3.66 m) and may have weighed about 500 pounds (226.80 kg). Although the chicken-sized kiwis w e r e similarly isolated in New Zealand, they never t e n d e d toward giantism; instead they reverted to a forest habitat w h e r e great size would have b e e n only a handicap. Unlike the ratites, penguins retained the keeled sternum and flight muscles because, in a sense, they simply readapted their locomotion from flying in the air to flying u n d e r water. Presumably, penguins evolved from flying aquatic birds—possibly stock ancestral also to the Procellariiformes (Simpson, 1946)—that reached the cool water ringing the Southern H e m i s p h e r e . Besides short, bladelike wings, or "flippers," suitable for propulsion, penguins acquired correlated structural features such as a streamlined or torpedoshaped body with legs so far back that they must stand upright when walking; feathering almost scalelike in aspect, without apteria; and a thick skin over a heavy layer of fat that provides insulation against the cool environment. A few species of penguins eventually ad-

justed to the more frigid water adjacent to Antarctica and soon nested exclusively on its periphery and outlying islands. The auks, which tend to fill the niche in the Northern H e m i s p h e r e occupied by penguins in the Southern, struck a compromise in their adaptation (Storer, 1960a). From presumed gull-like ancestors, all species, with one exception, derived wings small enough for underwater swimming, yet large enough for aerial flight. The exception was the Great Auk (Pinguinus impennis); like the penguins, it forsook the air altogether for submarine flight.

The Decrease of Birds Over the ages since life began, species of animals and plants have arisen, flourished, and died out. This applies as much to birds as to other forms. From the fossil evidence and theoretical knowledge of environmental conditions that existed during the long history of birds, one can easily surmise that there were far greater numbers of species and individuals in past ages than at the present time; but, in any attempt to d e t e r m i n e numbers one is frustrated by the meagerness of the record. Speculation is the only recourse. The rapid multiplication of bird species in the Eocene was favored not only by the decline of reptiles and the consequent availability of their habitats but also by the persisting warm climate and the increasing development of seed plants (angiosperms) that formed immense forests, creating many new niches for occupancy. By the time of the Oligocene, mountains had begun to rise, and as the land dried in their lee, forests soon decreased and grasslands formed. H e r e again were new habitats— in the mountains and on the plains—for avian radiation. Through the Miocene and into the Pliocene there was a slight cooling of the climate although it stayed warm or temperate much farther north than at the present time and there were no seasonal changes. But in the Pleistocene and its succession of four glacial stages, with the long and warm interglacial periods, the relative uniformity of the climate e n d e d and its effects on bird life were catastrophic. With each invading ice sheet, the prevailing t e m p e r a t u r e lowered. Birds were forced to

The Decrease of Birds shift and sometimes compress their ranges southward. Failing to cope with such radical changes, some species died out. How many species of birds have been identified from fossil remains? Roughly 1,700, according to Austin (1961). Of this n u m b e r , about 800 are still in existence and 900 are extinct. Adding the 900 extinct species to the 9,000 or so species known to be living in the world today, the total is about 9,900. Taking a backward look at the long history of birds, one can readily see an overall increase in the n u m b e r of species from the Late Jurassic to the Late Tertiary. In this great period of time the rate of increase exceeded the rate of extinction. If there was an Age of Birds, it must have been from the Miocene through the Pleistocene, a period of some 20,000,000 years. Since the Pleistocene, the rate of extinction is p r e s u m e d to have exceeded the rate of increase. The extent to which prehistoric man contributed to the decrease was probably negligible. As Greenway (1967) fancifully stated, "man and birds arranged a means of living together to the ends that no birds were extirpated." But there is no denying that in historic times man has played an awesome role in the extinction of birds. The first species actually known to be eliminated by man is the Dodo (Raphus cucullatus) on the island of Mauritius in the Indian Ocean. In the 174 years following the discovery of the island by the Portuguese in 1507, m e n from Europeon ships, and the cats, pigs, monkeys, and rats that they brought with them, succeeded in destroying the entire population. It was nothing less than miraculous that this flightless, ground-nesting species survived as long as it did. The Great Auk was the first species on the coast of North America that man annihilated. Breeding, probably in colonies, on rocky, coastal islands in the North Atlantic, this flightless bird was readily accessible to roving sailors and fishermen. They took its eggs for food and slaughtered it for meat, feathers, oil, and codfish bait. The last two specimens of the Great Auk were taken on Eldey, a volcanic rock off the southwest coast of Iceland, on June 3, 1884. At about this time on mainland North America two species, the Carolina Parakeet (Conuropsis carolinensis) and the Passenger Pigeon (Ectopistes

I 359

migratorius), were on their way to extinction through excessive killing, but nobody yet realized it. Both species lived east of the Great Plains; both were gregarious, commonly existing in large flocks. The Carolina Parakeet, though perhaps never very abundant, was especially fond of fruit and consequently much despised by farmers who could kill large n u m b e r s easily. W h e n a flock raided an apple tree, it was possible to shoot every bird because those individuals escaping the first blast from the gun hovered over those killed until they too were shot. The last specimen was killed in the wild on April 18, 1901. The Passenger Pigeon (Fig. 57), at the time of the white man's arrival in North America, may have n u m b e r e d 3,000,000,000 individuals—a popultion never attained by any other bird species known—and constituted between 25 and 40 percent of the bird population in the United States (Schorger, 1955). Despite this apparent security in n u m b e r s , the species was wiped out in the course of a century, the last wild specimen being recorded with certainty b e t w e e n September 9 and 15, 1899. By a remarkable coincidence, both the last captive Carolina Parakeet and the last captive Passenger Pigeon died in the same place in the same month and year—in the Cincinnati Zoological Garden in September, 1914 (Greenway, 1967.) The dramatic decline of the Carolina Parakeet and Passenger Pigeon in the last century eclipses the demise of another North American species, the Labrador Duck (Camptorhynchus labradorius), the last recorded specimen of which was taken in the fall of 1875. It was never an abundant bird and it was never h u n t e d extensively. Just what caused its extinction will never be known with any certainty. In this century the one remaining Heath Hen (Tympanuchus cupido cupido), the eastern subspecies of the Greater Prairie-Chicken, was last seen on March 11, 1932. Once prevalent along the Atlantic seaboard from New Hampshire south to Virginia, the Heath H e n became confined after 1869 to Martha's Vineyard, an island off the coast of Massachusetts. H e r e it survived in varying numbers, reaching a population close to 2,000 by 1916. But in the spring of that year a severe fire swept its breeding grounds, no doubt destroying many nests and nesting sites (Gross, 1928). In any case its final decline began soon thereafter and was accelerated

360 / Ancestry, Evolution, and Decrease of Birds

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Figure 57

Passenger Pigeon

in the few remaining years by predation from cats and rats, diseases acquired from poultry, and toward the end, an excessive ratio of males to females. All told since 1681, the last year when the Dodo was alive, no fewer than 78 species and 49 wellmarked subspecies have become extinct over the world. Fisher (1964), who compiled these figures, found "fairly strong direct p r o o f that man destroyed nearly half of the 78 species. Although Fisher makes no statement as to m a n s role in the destruction of the 49 subspecies, it was undoubtedly as great, if not even greater. Fisher attributes the causes of extinction by man

to b e primarily direct killing, destruction of habitat, and predation by cats, rats, and other human symbionts. Another cause, but difficult to prove, is man's introduction of competing bird species. Worthy of note from F i s h e r s figures is that only nine of the 78 extinct species and two of the 49 extinct subspecies were continental; all the others lived on islands. This clearly reflects the fact that insular birds, normally with small populations, are particularly sensitive to changes. Sometimes flightless, often quite tame, and usually with quite specialized feeding habits, they tend to lack the versatility to escape from, compete with, or adjust

References to man-imposed modifications in their natural environment. Besides those species and subspecies known to b e extinct are a far greater n u m b e r verging on extinction and, in some cases, may already b e extinct. The following four North American species and two subspecies have wild populations under 100: Species California Condor, Gymnogyps californianus Whooping Crane, Grus americana Eskimo Curlew, Numenius borealis Bachman's Warbler, Vermivora bachmanii Subspecies Florida Snail Kite, Rostrhamus sociabilis plumbeus American Ivory-billed Woodpecker, Campephilus p. principalis (probably extinct) For other bird species and subspecies in North America and elsewhere that may be nearing extinction, consult Endangered Birds of the World: The ICBP Bird Red Data Book (1979; reprinted in handbook form, 1981), compiled by W . B . King on behalf of the International Council for Bird Preservation and the Species Survival Commission of the International Union for Conservation of Nature and Natural Resources. Now and then a bird, believed to be extinct, has b e e n found still existing in some unexplored area or has simply b e e n overlooked. For example, the Takahe, Notornis mantelli, a heavy, flightless gallinule that once lived over much of New Zealand's South Island was not reported after 1898 until rediscovered exactly fifty years later in a remote mountain valley (Williams, 1960). The rare Puerto Rican Whip-poor-will or Nightjar, Caprimulgus noctitherus, described from bones found in prehistoric cave deposits and from a single specimen collected in 1888, and seen alive only once—in 1911— thereafter went unreported by ornithologists until fifty years later, when it was rediscovered by a taperecording of its call and from a collected specimen (Reynard, 1962). The giant race of the Canada Goose, Branta canadensis maxima, which breeds in the northern Great Plains, was rediscovered in 1962 after being considered extinct for three decades (Hanson, 1965). While it is not impossible that even a few other supposedly extinct birds will some day show up, it

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is only wishful thinking to expect that many are still extant. The truth must be faced: most of the species declared extinct are in fact gone forever. Will other birds become extinct? Undoubtedly. Species have apparently been decreasing since the Pleistocene, and no species can live on indefinitely. Indeed, every species has a normal life expectancy—perhaps as short as 16,000 years (calculation by Fisher, 1964; based on Brodkorb, 1960). The disturbing problem confronting ornithologists and other people interested in birds is how to blunt the human threat that may shorten life expectancy and hasten extinction. The surging h u m a n population, now estimated at 3 billion and expected to be doubled by the year 2000, is not in itself a threat to the longevity of bird species. The threat comes in man's thoughtless abuse of the earthly environment. Promiscuous dissemination of poisons, pollution of air and water, total destruction rather than the selective use of habitats, indiscriminate creation of hazards to bird migration—all such actions, if they increase as the h u m a n population expands, can in a short time eliminate bird species by the score. The main hope for birds lies in more aggressive conservation of natural environments. Everyone interested in ornithology should become militant conservationists and apply their newly acquired knowledge toward countering the threat to the early extinction of birds.

References Austin, O.L., Jr. 1961 Birds of the world. New York: Golden Press. Bock, W J . 1963 The cranial evidence of ratite affinities. Proc. XIHth Internatl. Ornith. Congr. pp. 39-54. Brodkorb, P. 1960 How many species of birds have existed? Bull. Florida State Mus., Biol. Sei. 5:41-53. de Beer, G. 1954 Archaeopteryx lithographica: A study based upon the British Museum specimen. London: British Museum (Natural History). 1964 Archaeopteryx. In A new dictionary of birds, ed. A.L. Thomson, New York: McGraw-Hill. Feduccia, A. 1980 The age of birds. Cambridge: Harvard Univ. Press.

362 / Ancestry, Evolution, and Decrease of Birds Fisher, J. 1964 Extinct birds. In A new dictionary of birds, ed. A.L. Thomson. New York: McGraw-Hill. Fisher, J., and Peterson, R.T., [1964] The world of birds. Garden City, New York: Doubleday. Greenway, J.C., Jr. 1967 Extinct and vanishing birds of the world. 2nd ed. New York: Dover. Gross, A.O. 1928 The Heath Hen. Mem. Boston Soc. Nat. Hist. 6:487-588. Hanson, H.C. 1965 The Giant Canada Goose. Carbondale: Southern Illinois Univ. Press. Heilmann, G. 1927 The origin of birds. New York: D. Appleton. Mengel, R.M. 1964 The probable history of species formation in some Northern Wood Warblers (Parulidae). Living Bird 3-43. Millener, P.R. 1982 And then there were twelve: The taxonomic status of Anomalopteryx eweni (Aves: Dinornithidae). Notornis 29:165-170. Parkes, K.C. 1966 Speculations on the origin of feathers. Living Bird 5:77-86. Reynard, G.B. 1962 The rediscovery of the Puerto Rican Whippoor-will. Living Bird 1:51-60. Savile, D.B.O. 1957 The primaries of Archaeopteryx. Auk 74: 99-101.

Schorger, A.W. 1955 The Passenger Pigeon: Its natural history and extinction. Madison: Univ. of Wisconsin Press. Simpson, G.G. 1946 Fossil penguins. Bull. Amer. Mus. Nat. Hist. 87:1-99. Storer, R.W. 1960a Evolution in the diving birds. Proc. Xllth Internatl. Ornith. Congr. pp. 694-707. 1960b Adaptive radiation in birds. In Biology and comparative physiology of birds, vol. 1, ed. A.J. Marshall. New York: Academic Press. Swinton, W.E. 1960 The origin of birds. In Biology and comparative physiology of birds, vol. 1, ed. A.J. Marshall. New York: Academic Press. 1964 Fossil birds. In A new dictionary of birds, ed. A.L. Thomson. New York: McGraw-Hill. Thulborn, R.A., and Hamley, T.L. 1982 The reptilian relationship of Archaeopteryx. Australian Jour. Zool. 30, 611-634. 1959 Birds of the Pleistocene in North America. Smithsonian Misc. Coll. 138:1-24. Wetmore, A. 1955 Paleontology. In Recent studies in avian biology, ed. A. Wolfson. Urbana: Univ. of Illinois Press. 1956 A Check-list of the fossil and prehistoric birds of North America and the West Indies. Smithsonian Misc. Coll. 131 (5):1-105. Williams, G.R. 1960 The Takahe (Notornis mantelli, Owen, 1848): A general survey. Trans. Royal Soc. New Zealand 88:235-258.