TEXT` Basic sets of tested hypotheses, combined into theories, support the scientific study of birds (and all living organisms). Evolution is the chief among these. Although details of its mechanisms and rates are still under study, in essence the theory of evolution holds that different species have descended, over time, with modifications, from previously living forms. The great group of factors influencing these changes is called ╥natural selection.╙ All animals compete for limited resources such as food, living space, and mates, and strive in changing environments. Individual organisms and interactive groups of organisms that are better able to compete will survive to reproduce and pass on genetically-based abilities to their descendants. Organisms that lose out in competition or fail to adjust to environmental changes will not reproduce successfully and their traits will be lost. Some of these losses are reflected in the fossil record as extinctions. Surviving traits are said to be ╥naturally selected.╙ The resulting adaptations to competition and to environmental pressures (e.g., climate) are multiple. They are behavioral, physiological, anatomical, biochemical--and all are ultimately based in genetic codes specific to an individual but shared in general with close relatives.SYSTEMATICSScientists that classify animals and study how they are related to each other are called ╥systematists╙ and their field of study is called ╥systematics.╙ A proper classification of any group of animals is arranged in a hierarchy that reflects the current state of knowledge about how the different groups are related to one another. Species included in a single genus are very closely related; species in different families (but within the same order) are less closely related; species in different orders are only distantly related. Systematists also try to arrange classification schemes to reflect their ideas about the evolutionary history of the entire group. ╥Primitive╙ groups have characters believed to resemble an ancient ancestor; groups with ╥advanced╙ characters are believed to have evolved more recently. Systematists arrange their classification hierarchies in branched trees, or cladograms, to indicate relationships and evolutionary history.Although there can be sub-categories, the major classification divisions are Phylum, Class, Order, Family, Genus, and Species.The primary building block of this hierarchy is the species. What is a species? Most people can recognize the members of a species just by looking. A Mallard is a Mallard, although a naive observer might classify the multi-colored males in a separate group from the brown females. Biologists define a ╥species╙ as an interbreeding population of genetically related organisms whose young (when adult) are essentially similar to the parents, and which retains genetic integrity from generation to generation. (Individuals of some species are known to sometimes interbreed with other species--hybridize--in abnormal conditions such as captivity or a disturbed environment. But species overwhelmingly maintain their specific genetic identity in nature and do not interbreed.)Consider as an example the taxonomic classification of the American Crow:Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Aves Order: Passeriformes Family: Corvidae (Crow) Genus: ╥Corvus╙ Species: ╥brachyrhynchos╙To make determinations about relationships, systematists use characteristics believed to reflect genetic similarities. The more closely related two animals are, the more similar their genetic codes. For centuries, systematists relied solely on anatomical characters to infer genetic relationships. Today, scientists are increasingly able to look directly at genetic codes through DNA and protein analyses. However, most studies of bird taxonomy (the subset of systematics that focuses on classification) rely on suites of characters: anatomical, behavioral, and biochemical. Determination of higher relationships, i.e., at the family and order level, and the evolutionary history of a group must rely in part on the study of fossil forms. Paleontology is essential in supporting ideas about relationships at higher taxonomic levels. Similarities to fossil forms may reflect ancestral lines.Convergence and DivergenceReliance on superficial, external similarities such as coloration, shape, size, and habits can mislead taxonomists. Some unrelated or distantly related birds may outwardly appear very similar, such as auks and penguins. Conversely, closely related birds may look very different, such as crows and birds of paradise.Convergence is the effect of two different species responding to similar selection pressures with resulting similarities in form and behavior. One striking example of convergence is seen in Western and Eastern meadowlarks (╥Icteridae╙) and African pipits (╥Motacillidae╙). While genetically unrelated, both genera build domed nests on the ground, live in similar habitats, and have very similar color patterns. Divergence results from adaptive radiation. This occurs when colonizing populations expand into an area where feeding opportunities (or other ecological niches) are diverse and free of competition. The classic example is presented by the Darwin╒s finches in the Galapagos Islands. When a small population of finch-like birds arrived in the unoccupied islands, a variety of new ways of making a living were open to them. Over thousands of years flocks of birds became separated on individual islands and exploited different foods and habitats. Over time, the populations diverged in form, or adaptively radiated. Darwin found related species with greatly differing bill shapes, for example, from parrot-like to woodpecker-like.Good taxonomy uses a variety of relatively stable characteristics, from external morphology likebill shapes and foot scales to internal anatomy like skull structure and throat musculature, as well as biochemical characters.FOSSIL BIRDSTo probe the origins of modern bird species, scientists commonly begin with fossils. Compared with other vertebrate groups, such as fishes, however, relatively few complete fossilized skeletons of birds have been discovered in the world. For the most part, only single bones and fragments have been found, representing the larger, denser ends of wing and leg bones or parts of the pectoral girdle--the part of the bony skeleton that supports the wings. Bird bones are lightweight and fragile, so they are easily and quickly destroyed and rapidly decompose. Bird fossils have been found mainly in protected places like caves, or under the bottoms of dried-up lakes, in fine-grained limestone and sandstone strata, and in bogs, rock quarries, and tar pits.ArchaeopteryxThe most significant fossil find in bird biology--and up to the present time, perhaps the most ancient bird (some would say bird-like reptile) yet known--is ╥Archaeopteryx lithographica.╙ Fossil evidence comes from an isolated feather, and two nearly complete skeletons found about ten miles apart in a quarry in Bavaria. The limestone quarries at Solnhofen had once been the sandy bottom of a shallow sea or lagoon in the Jurassic Period about 130-160 million years ago. This very fine-grained sandstone permitted the formation of detailed fossils and also produced excellent material for nineteenth century lithographers, hence the species name and the active quarrying of material.Archaeopteryx was a reptile-like bird, about the size of a crow. Fossils show the clearly marked imprint of feathers attached to the forearm. It has been theorized that it lived around trees, judging by the position of its big toe (╥hallux╙), placed opposite the other three, presumably adapted to gripping a perch. The broad, rounded, galliform wings of Archaeopteryx had six or more primary feathers attached to the hand and wrist part of the wing, and secondary feathers attached to the forearm. Modern birds have nine to twelve primaries, depending on the species.Archaeopteryx lacked the hollow bones typical of modern birds, and its powers of flight are assumed to have been limited. The lack of a keel on its breastbone suggests it had no powerful flying muscles. (The flight muscles of modern birds are attached to this keel.) Its wings differed from those of all modern flying birds in many features necessary to powered flight: unfused flight bones (╥manus╙ and ╥metacarpus╙); unrestricted wrist and elbow joints; unmodified coracoids and the lack of a keeled breastbone. The wings of Archaeopteryx may have been better adapted to catching prey than for flight.It is likely that the feathers of Archaeopteryx evolved in conjunction with a high level of activity and metabolism, and a need to control or minimize heat loss. Some scientists believe that Archaeopteryx was a ground-dwelling, bipedal, running predator similar to the modern secretary-bird. Others believe instead that these early birds were tree dwellers.These ideas are related to two main theories of the origin of flight. One holds that early flight evolved from long-tailed bipedal reptiles that flapped their forelimbs as they ran rapidly over the ground. The other hypothesizes that ancient bird relatives glided from tree to tree or from tree toground. There is an enormous gap in the record of fossil birds between the time of Archaeopteryx in the Jurassic Period and the next great discoveries. However, fossils show that most modern orders of birds were well-differentiated by the Eocene epoch, some 60 million years ago.Fossil Birds of North AmericaThe first record of a fossil North American bird was a written account in 1834 of the leg bone (╥tibia╙) of a Cretaceous (120 million years ago) snipe-like bird found buried in a New Jersey marl bed. Nearly complete bird skeletons have been found in marine Cretaceous chalk beds in Kansas. One such bird was ╥Hesperornis,╙ a flightless, loon-like bird nearly five feet long. In the same formations were found gull-like birds with well-developed wings and strong sternums. These fossils, named ╥Ichthyornis╙ (a genus with seven or so species), were found in association with many fossil fishbones, apparently reflecting the diet of the coastal, marine birds.Lower Eocene formations (60 million years ago) in Wyoming, New Mexico, and New Jersey have yielded fossil bird skeletons of great size. ╥Diatryma giganteum,╙ from New Mexico, was 7 feet tall with a head as big as that of a horse.CLASSIFICATIONNotesSo-called scientific, or Latin, names of birds may look technical and irrelevant to the casual reader, but they contain much useful information about the creatures to which they are assigned. It is well worthwhile for anyone with an interest in nature to learn just a bit about scientific classification. The ╥naming of a new species╙ is not a casual activity but is fraught with responsibility and is highly structured. All scientists accept two books of rules for the naming of living organisms, ╥The International Code of Zoological Nomenclature╙ and the ╥International Code of Botanical Nomenclature.╙ These rules are complicated but are essentially based on three principles: 1) Every organism shall be named in a consistent way that reflects its relationship with other organisms; 2) No two kinds of animals or plants shall have the same scientific name;3) In the accidental event that the above principle is violated, the animal that has had the name the longest shall keep its name and the other shall get a new name. Because of universal acceptance of these rules, scientific names are standardized all over the world. Common names are easy to understand, and are often very descriptive (if you happen to speak the language in which the name is expressed). However, they are not universally standardized, and may vary from one locality to another, or even from one person to another.A well-considered scientific name can contain a lot of information, but there are also some nuances. Although officially discouraged, ╥patronymics╙ are often created to honor colleagues or collectors. Cooper╒s Hawk (╥Accipiter cooperi╙); the Spectacled Eider (╥Somateria fischeri╙); the Yellow-billed Loon (╥Gavia adamsii╙); and the Gray Flycatcher (╥Empidonax wrightii╙), are examples. Note that in at least the last three cases, the common name tells us more about the bird╒s appearance than does the scientific name. Many taxonomists believe that naming a plant or animal after a place or person wastes the opportunity to include useful information in the name. Good examples of such useful common names are: ╥Buteo╙ (Latin for ╥small hawk╙) ╥platypterus╙ (Greek for ╥flattened, or broad wing╙)--the Broad-winged Hawk; and its close relative, ╥Buteo albicaudatus,╙ the White- (╥albi╙) tailed (╥cauda╙) Hawk. A little knowledge of Greek and Latin plus a good classical or biological dictionary help a lot in both understanding and remembering the scientific names of birds.The basic scientific name we usually see for an organism is comprised of two parts, the genus and the species. The genus (always first and always capitalized although sometimes abbreviated to its initial letter) represents a group of species believed by scientists who study them to be related to each other. Related genera (the plural for more than one genus) are classified into families. Usually family names end in the letters - ╥idae.╙ Related families are included within an order, with the customary ending of - ╥iformes.╙ Working with a variety of techniques ranging from morphology to biochemistry and the study of fossils, scientists group families into a hierarchy of classification that represents tentative conclusions about relationships and evolutionary history. With increased study and new knowledge, scientists may revise classification schemes and re-examine how groups may berelated to one another.Classification of North American BirdsThe following classification generally reflects current thought; only families with species in North America are listed here.Order I. Gaviiformes: This order contains only one small, uniform family, the loons (called divers in the Old World). These swimming, diving birds come ashore only to nest in marshes and on the borders of lakes and ponds. They are limited to the Northern hemisphere, nesting in boreal and Arctic regions and wintering somewhat farther south. Family: Gaviidae: loonsOrder II. Podicipediformes: This order is comprised of a single family of aquatic, small- to medium-sized birds called grebes. They are long-necked and sharp-billed with lobed toes, and propel themselves with their feet when diving. They are awkward on land and often build floating nests. The more northerly species in North America are highly migratory. Family: Podicipedidae: grebesOrder III. Procellariiformes: The ╥tube-noses╙ are found throughout the world and are most diverse in the southern hemisphere. They are the most strictly marine of birds found in North America. All are web-footed, with long wings and excellent flying abilities. As their ordinal name implies, these birds all have nostrils enclosed in tubes on the outside of the bill. Family Diomedeidae: albatrosses Family Procellariidae: shearwaters and petrels Family Hydrobatidae: storm-petrelsOrder IV. Pelecaniformes: Pelicans and their relatives are a diverse and world-wide group found mainly in coastal and marine habitats. All six families are represented in North America, but some are limited to tropical and sub-tropical areas. All pelicaniform birds have four toes connected by webbing, a unique character. Almost all species are adapted for eating fish. Family Phaethontidae: tropicbirds Family Pelecanidae: pelicans Family Sulidae: boobies and gannets Family Phalacrocoracidae: cormorants Family Anhingidae: anhingas or darters Family Fregatidae: frigatebirdsOrder V. Ciconiiformes: In North America this order is represented by three families of long-legged wading birds. The order is world-wide in distribution but is poorly represented in higher latitudes. Family Ardeidae: bitterns and herons Family Threskiornithidae: ibises and spoonbills Family Ciconiidae: storks Family Phoenicopteridae: flamingosOrder VI. Anseriformes: Ducks, geese, and swans are collectively referred to as waterfowl. The group is world-wide in distribution. Most species that breed in high latitudes are highly migratory, often sexually dimorphic (i.e., the sexes are differently plumed), and form large flocks on the wintering grounds. Family Anatidae: swans, geese, and ducksOrder VII. Falconiformes: Nearly all are diurnal birds of prey or carrion feeders, and have acute vision, sharp-hooked beaks, and powerful talons. The order is found world-wide. Many species are highly migratory. Family Cathartidae: American vultures Family Accipitridae: kites, hawks, and eagles Family Falconidae: caracaras and falconsOrder VIII. Galliformes: This group is sometimes referred to as gallinaceous or ╥fowl-like╙ birds, indicating the inclusion of familiar species used as food by humans. The order is nearly world-wide in distribution and generally consists of strong-legged, stout-billed, terrestrial, or partly arboreal birds with a wide range of sizes. Family Cracidae: curassows, guans, and chachalacas Family Phasianidae: partridges, grouse, turkeys, and quail Order IX. Gruiformes: This group shows a world-wide diversity in body type and size. One author describes them as variously resembling ╥chickens, herons, ducks, and shorebirds.╙ North American representatives usually live in marshes and related aquatic habitats. Family Rallidae: rails, gallinules, and coots Family Aramidae: limpkins Family Gruidae: cranes Order X. Charadriiformes: This large and diverse world-wide order includes several distinct adaptive lines, reflected in the classification into four suborders. All the birds within these groups live on or near water. Most of them use their variously adapted bills to probe or pick for food. Most are long legged. Birds in the first two suborders are migratory, nesting in the Arctic and undertaking long-distance migrations to southern wintering grounds. Suborder Charadrii Family Charadriidae: plovers and lapwings Family Haematopodidae: oystercatchers Family Recurvirostridae: avocets and stilts Suborder Scolopaci Family Jacanidae: jacanas Family Scolopacidae: sandpipers, phalaropes, and allies Suborder Lari: The single family within this suborder now includes some forms previously categorized in separate families. All Lari are web-footed, long-winged, and aquatic. The group employs a variety of feeding techniques. Many species scavenge. Jaegers and skuas are terrestrial scavengers or rodent hunters during the breeding season, but pirate food from seabirds during non-breeding periods. Terns plunge-dive for food; skimmers skim the surface for fish. Family Laridae: jaegers, skuas, gulls, terns, and skimmers Subfamily Larinae: gulls Subfamily Sterninae: terns Subfamily Stercorariinae: skuas and jaegers Subfamily Rynchopinae: skimmers Suborder Alcae: The single family in this suborder includes various small-to-medium-sized diving seabirds with stubby bodies, small wings, webbed feet, and variously shaped bills, often ornamented during breeding season. Family Alcidae: auks, murres, and puffinsOrder XI. Columbiformes: Old world families include sandgrouses and show characters that appear to relate this group to the extinct dodos and relatives. North American pigeons and doves are fruit and seed eaters. Most forage on the ground, although some feed in trees. Family Columbidae: pigeons and dovesOrder XII. Psittaciformes: This large family is found throughout world tropics and subtropics. The only species widespread in North America, the Carolina parakeet, is now extinct. Exotic species have escaped from pet cages and have established feral populations in warmer parts of North America. Family Psittacidae: parrotsOrder XIII. Cuculiformes: Although cuckoos and their relatives are found in temperate zones and tropical areas around the world, only one family of this order occurs in North America. The family is represented by drably plumed, woodland-dwelling cuckoos; the arid-country, longer-legged roadrunner; and subtropical brushland anis. Family Cuculidae: cuckoos, roadrunners, and anisOrder XIV. Strigiformes: The distinguishing characteristics that unite the species in the owl order are among the most obvious of the bird orders. The large, forwardly-directed eyes set in a feathered disk and the strong-curved beaks are related to their feeding habit of nocturnal hunting. Family Tytonidae: Barn Owl Family Strigidae: true owlsOrder XV. Caprimulgiformes: Of the five families in this order, only one has representatives in North America. These nightjars are long-winged insect hunters that hunt at dawn and dusk (called crepuscular feeding). Family Caprimulgidae: nightjarsOrder XVI. Apodiformes: The name of this unusual group comes from a root meaning ╥without feet.╙ While not anatomically true, it emphasizes the extraordinary flying abilities of swifts and hummingbirds. The swift family is found over most of the world. Hummingbirds are restricted to the New World with most of their diversity in the tropics. Both families have small feet, short legs, and pointed wings. The primary wing feathers are long and the secondaries relatively few, so the bend of the wing lies close to the body. Family Apodidae: swifts Family Trochilidae: hummingbirdsOrder XVII. Trogoniformes: The single family in this order has representatives throughout most of the world╒s tropics. Only two species reach the southern extremes of North America. All are colorful, stout-billed, long-tailed forest birds that generally feed on insects and fruits. Family Trogonidae: trogonsOrder XVIII. Coraciiformes: This diverse group of nine families includes such outwardly different birds as hornbills (Africa, tropical Asia), bee-eaters (Old World temperate and tropics), and the North American representatives, kingfishers. All are united by the anatomical character of ╥syndactyly,╙ the fusion of toes for a considerable part of their length. Family Alcedinidae: kingfishersOrder XIX. Piciformes: Of the seven families in this order, the best known, and the only North American representatives, are the woodpeckers. The others (e.g., toucans, jacamars, barbets, honeyguides) occur in wooded areas nearly world-wide, although they are lacking in Australasia and oceanic islands. Woodpeckers have ╥zygodactylous feet,╙ i.e., two toes point forward and two (rarely only one) extend back. Family Picidae: woodpeckersOrder XX. Passeriformes: More than half the world╒s bird species belong to the ╥perching bird╙ order. They show a tremendous diversity in feeding adaptations and form the preponderance of bird species in terrestrial communities. All passeriforms have perching feet: three toes extend forward and one extends backward, all at the same level. Family Tyrannidae: tyrant flycatchers Family Alaudidae: larks Family Hirundinidae: swallows and martins Family Corvidae: crows, magpies, and jays Family Paridae: titmice Family Remizadae: verdins Family Aegithalidae: bushtits Family Sittidae: nuthatches Family Cathode: creepers Family Pycnonotidae: bulbuls Family Troglodytidae: wrens Family Cinclidae: dippers Family Muscicapidae: Old World warblers and thrushes Subfamily Sylviinae: Old World warblers, kinglets, and gnatcatchers Subfamily Turdinae: thrushes Family Mimidae: mockingbirds and thrashers Family Prunellidae: accentors Family Motacillidae: wagtails and pipits Family Bombycillidae: waxwings Family Ptilogonatidae: Silky-Flycatcher Family Laniidae: shrikes Family Sturnidae: starlings and mynas Family Vireonidae: vireos Family Emberizidae: warblers, grosbeaks, and sparrows Subfamily Parulinae: wood warblers Subfamily cobrebinae: Bananaquit Subfamily Thraupinae: tanagers Subfamily cardinalinae: cardinals, grosbeaks, and allies Subfamily Emberizinae: New World sparrows and allies Subfamily Icterinae: New World blackbirds and allies Family Fringillidae: finches, canaries, and siskins Family Passeridae: Old World sparrows
