Birds, the class Aves, as generally understood by systematic zoologists, but of late years classified with the reptiles in one large order, Sauropsida of Huxley. Although apparently so different in external appearance from the Reptilia, birds are but highly modified reptiles, when the characters of their osteology and comparative anatomy are taken into account. The chief outward difference consists in the fact that birds have feathers, which no reptile possesses. Their young, likewise, are hatched from eggs, but this is by no means a character peculiar to birds, for it is now known that among the mammalia the Ornithorhynchus produces its young from an egg, while turtles and crocodiles and many snakes also lay eggs. Birds may, therefore, be described as warm-blooded, oviparous, vertebrate animals, clothed with feathers.
The earliest fossil remains of any form of bird have been found in the Jurassic rocks of Bavaria (Archaeopteryx); they have also been discovered in the Cretaceous, Eocene, Miocene, and all the later deposits both of this country and abroad.
It has been ascertained beyond all doubt that the most ancient birds possessed teeth, and that the feathers, though veritable plumes, were not quite of the same character as those observed in the birds of the present epoch. Thus the Archaeopteryx, the wonderful fossil form of extinct bird-life discovered in the lithographic slate of Solenhofen in Bavaria, had an enormously long tail, exceeding the length of the body itself, and furnished with lateral plumes along its entire extent. Hence it has been proposed by Professor Gill in America to divide birds into two main divisions, one of which would comprise the lizard-tailed Saurura, represented by Archaeopteryx, while the great mass of birds would be called Euriphidurae, or fan-tailed birds, wherein the tail is spread, or at least arranged, on the plan of a fan. Two other groups of birds are recognised by naturalists, the Odontormae and the Odontooleae, both represented by extinct forms, which also possessed teeth.
In the time of Linnaeus, and for a generation or two afterwards, the class "Aves" was arranged according to external and visible characters only. Thus an early plan was to separate the feathered tribes into "Land" birds and "Water" birds. Then followed the division into raptorial birds, perching birds, game birds, wading birds, swimming birds, etc., with many subdivisions such as fissirostral or wide-gaping birds, scansorial or climbing birds, etc. But as the study of science advanced many other characters were found to be of importance; for instance, the pterylography or arrangement and structure of the feathers, the shape of the sternum, and the general osteology. A great influence for good was exercised by the publication of Darwin's Origin of Species, and the geographical distribution of birds began to be zealously studied. In 1867 Professor Huxley published his classification of birds, in which many previously unknown characters were brought to light, and this important publication underlies all the recent systematic work of ornithologists who have attempted to arrange the class "Aves." Much has been done since by Parker, Garrod, Forbes, Furbenger and Gadow, to add to Huxley's foundation; and in all recent arrangements of the birds, osteological and anatomical characters have been chiefly relied on, somewhat to the neglect of the external form and the habits of species, which are also of equal importance in determining what the affinities of a bird really are.
Huxley divides the class "Aves" into three large orders: -
1. Saururae (lizard-tailed birds - the fossil Archaeopteryx).
2. Ratitae (flightless birds which have no keel to the sternum - Ostriches, Rheas, Emeus, Apteryx).
3. Carinatae. All the remaining families of birds which possess a keel to the sternum.
An exception is seen in the case of the owl-parrot of New Zealand (Stringops habroptilus), which has completely lost the power of flight, so that the keel of the sternum, being no longer of use for the attachment of the pectoral muscles, has become in process of time obsolete.
The Ratitae consists of the ostrich and its allies, i.e. the struthious birds as they are generally called. By many systematists they are considered to be the most ancient type of bird which survives at the present day, and are supposed to indicate the forerunners of all the forms of bird-life now on the earth. That they are of ancient origin is undoubted, but it is more probable that they point to an early departure from the reptile-like birds of a long past epoch. They apparently spring from a stock which once had amply developed wings, which through disuse have gradually become aborted, development of the legs and running power being correspondingly increased. It has been stated that in the embryo ostrich the development of the wings proceeds at first as in other birds, but that after a time the growth ceases and the development of the legs proceeds at the expense of the wings. The kiwis (Apteryx) of New Zealand also belong to the struthious birds according to their osteology, but in habits they are akin to rails (Ralli).
Of the carinate birds, the Tinamous (Crypturi, or Tinami) have a struthious palate, in which the vomer is united in front of the broad maxillo-palatine plates, as in the Emeu; while its shape and attachment behind is also like that of the struthiones. This peculiarity has induced Huxley to call the Tinamous "Dromaeognathous."
A second arrangement of the palatine bones is called by Huxley "Schizognathous." In these birds the vomer tapers to a point anteriorly, and divides the maxillo-palatine bones, which in consequence do not coalesce. Such are plovers, gulls, etc.
The third arrangement of the palatine bones is "Desmognathous," and here the vomer tapers to a point anteriorly, but the maxillo-palatines are united across the middle line. Hawks, ducks, etc., are characteristic Desmognathous birds.
Lastly, the great mass of passerine birds have an "AEgithognathous" palate, intermediate in type between the "Schizognathous" and "Desmognathous" forms. The vomer is truncated in front, and deeply-cleft posteriorly, so as to embrace the sphenoidal rostrum. The maxillo-palatines do not unite with each other or with the vomer.
As before mentioned, Huxley's Dromaeognathae contain only the tinamous. The Schizognathae include all the plovers and gulls, cranes, rails, petrels, divers, grebes, penguins, game-birds, and pigeons. The Desmognathae comprise all the herons, storks, ducks, flamingoes, pelicans and allies, birds of prey, parrots, and the bulk of what are known as Picarian birds (cuckoos, kingfishers, trogons, etc.). The AEgithognathae contain the passerine birds, with the swifts, humming-birds, and goatsuckers.
Several classifications of birds have been proposed since Huxley's time, but none have produced such important alterations in the line of study. The most celebrated is that of Furbringer, which is the result of many years of labour, and is the most comprehensive work on the anatomy and morphology of birds. Dr. Bowdler Sharpe has recently passed in review all the schemes of classification published during the last twenty-five years, and, as a result, he has proposed to the meeting of the second Ornithological Congress at Budapest, held in May, 1891, the following linear arrangement: -
Sub-class I. - Saururae.
Order I. - Archieopteryx (fossil).
Sub-class II. - Ratitae.
Order II. - Rheiformes (Rheas).
Order II. - Struthioniformes (Ostriches).
Order IV. - Casuariiformes (Casowaries and Emeus).
Order V. - Apteryges (Kiwis).
Sub-class III. - Carinatae.
Order VI. - Crypturiformes (Tinamous).
Order VII. - Galliformes (Game-birds).
Sub-order Megapodii (Megapodes).
Sub-order Craces (Curassows).
Sub-order Phasiani (True Game-birds).
Family Phasianidae (Pheasants).
Family Tetraonidae (Grouse).
Family Perdicidae (Partridges).
Family Numididae (Guinea-Fowls).
Family Meleagridae (Turkeys).
Sub-order Hemipodii (Hemipodes).
Sub-order Pterocletes (Sand-Grouse).
Order VTII. - Columbiformes (Pigeons).
Order IX. - Opisthocomiformes (Hoatzins).
Order X. - Ralliformes (Rails).
Family 1. - Gallinulidae (Water-Hens).
Family 2. - Railidae (True Rails).
Family 3. - Ortygometridae (Craties).
Family 4. - Podicae (Fin-Foot).
Order XI. - Heliornithiformes (Sun-Grebe).
Order XII. - Podicipitidiformes (Grebes).
Order XIIT. - Colymbiformes (Divers).
Order XIV. - Sphenisciformes (Penguins).
Order XV. - Procellariiformes (Petrels).
Family 1. - Diomedeidae. (Albatrosses).
Family 2. - Procellariidae (True Petrels).
Family 3. - Pelecanoididae (Diving Petrels).
Order XVI. - Alciformes (Auks).
Order XVII. - Lariformes.
Family 1. - Stercorariidae (Skuas).
Family 2. - Laridre (Gulls and Terns).
Order XVIII. - Charadriiformes.
Sub-order Dromades (Crab-Plovers).
Sub-order Chionides (Sheath-bills).
Sub-order Attagides (Quail-Plovers).
Sub-order Charadril (True Plovers).
Family Haematopodidae (Oyster-catchers).
Family Charadriidae (Plovers).
Family Scolopacidae (Snipes).
Sub-order Glareolae (Pratincoles).
Sub-order Cursorii (Coursers).
Sub-order Parrae (Jacanas).
Sub-order OEdicnemi (Thick-knees).
Sub-order Otidides (Bustards).
Order XIX. - Gruifonnes.
Sub-order Grues (Cranes).
Sub-order Arami (Courlans).
Sub-order Rhinochetides (Kagus).
Sub-order Mesitides (Ground-Herons).
Sub-order Eurypygae (Sun-Bitterns).
Sub-order Psophiae (Trumpeters).
Sub-order Dicholophi (Seriamas).
Order XX. - Pelargiformes.
Sub-order Ardeae (Herons).
Sub-order Ciconii (Storks).
Sub-order Balaenicipitides (Shoe-bills).
Sub-order Seopi (Umbres).
Family Plataleidae (Spoonbills).
Family Ibididae (Ibises).
Order XXI. - Phoenicopteriformes (Flamingoes).
Order XXII. - Anseriformes.
Sub-order Anseres (Ducks and Geese).
Sub-order Palamedeae (Screamers).
Order XXIII. - Pelecaniformes.
Sub-order Phaethontes (Tropic-buds).
Sub-order Sulae (Gannets).
Family Phalacrocoracidae (Cormorants).
Family Plotidae (Darters).
Sub-order Pelecani (Pelicans).
Sub-order Fregati (Frigate-birds).
Order XXIV. - Cathartidiformes (Turkey Buzzards).
Order XXV. - Accipitriiformes.
Sub-order Serpentarii (Secretary-birds).
Family Vulturidae (Vultures).
Family Falconidae (Hawks).
Sub-order Pandiones (Ospreys).
Sub-order Striges (Owls).
Order XXVI. - Coraciiformes.
Sub-order Steatornithes (Oil-birds).
Sub-order Podargi (Frog-mouths).
Sub-order Leptososmati (Kirombos).
Sub-order Coraciae (Rollers).
Sub-order Halcyones (Kingfishers).
Sub-order Bucerotes (Hornbills).
Sub-order Upupae (Hoopoes).
Sub-order Meropes (Bee-eaters).
Sub-order Momoti (Mot-mots).
Sub-order Todi (Todies).
Sub-order Caprimulgi (Goatsuckers).
Sub-order Cypseli (Swifts).
Sub-order Trochili (Humming Birds).
Sub-order Colii (Colies).
Order XXVII. - Trogones (Trogons).
Order XXVIIL - Coceyges.
Sub-order Musophagi (Tourakoes).
Sub-order Cuculi (Cuckoos).
Order XXIX. - Psittaciformes.
Family Nestoridae (Nestors).
Family Loriidae (Lories).
Family Cyclopsittacidae (Lorikeets).
Family Cacatuidae (Cockatoos).
Family Psittacidae (True Parrots).
Family Stringopidae (Owl-Parrots).
Order XXX. - Scansores.
Sub-order Rhamphastides (Toucans).
Sub-order Capitones (Barbets).
Sub-order Indicatores (Honey-Guides).
Order XXXI. - Piciformes.
Sub-order Piei (Woodpeckers).
Sub-order Buccones (Puff-birds).
Sub-order Galbulae (Jacamars).
Order XXXII. - Menurae (Lyre-birds).
Order XXXIII. - Eurylaemi (Broad-Bills).
Order XXXIV. - Passeriformes.
Section A. - Oscines.
Family 1. - Corvidae (Crows).
Family 2. - Paradiseidae (Birds of Paradise).
Family 3. - Ptilonorhynchidae (Bower-birds).
Family 4. - Sturnidae (True Starlings).
Family 5. - Eulabetidae (Tree-Starlings).
Family 6. - Eurycerotidae (Blue-bills).
Family 7. - Dieruridae (Drongos).
Family 8. - Oriolidie (Orioles).
Family 9. - Icteridae (Hang-nests).
Family 10. - Ploceidae (Weaver-birds).
Family 11. - Tanagridae (Tanagers).
Family 12. - Caerebidae (American Creepers).
Family 13. - Fringillidae (Finches).
Family 14. - Alaudidae (Larks).
Family 15. - Motacillidae (Wagtail and Pipits).
Family 16. - Mniotiltidae (American Warblers).
Family 17. - Certhiidae (Creepers).
Family 18. - Meliphagidae (Honey-Suckers).
Family 19. - Diceidae (Flower-Peckers).
Family 20. - Zosteropidae (White-Eyes).
Family 21. - Paridae (Tits).
Family 22. - Regulidae (Gold-Crests).
Family 23. - Laniidae (Shrikes).
Family 24. - Artamidae (Swallow-Shrikes).
Family 25. - Ampelidae (Wax-wings).
Family 26. - Vireonidae (Greenlets).
Family 27. - Sylviidae (Warblers.)
Family 28. - Turdidae (Thrushes).
Family 29. - Cinelidae (Dippers).
Family 30. - Troglodytidae (Wrens).
Family 31. - Mimidae (Mocking Birds).
Family 32. - Timeliidae (Bush-Babblers).
Family 33. - Pyenonotidae (Bulbuls).
Family 34. - Campophagidae (Cuckoo-Shrikes).
Family 35. - Muscicapidae (Flycatchers.)
Family 36. - Hirundinidae (Swallows).
Section B. - Oligomyodi.
Family 1. - Tyrannidae (Tyrants).
Family 2. - Oxyrhamphidae (Sharp-bills).
Family 3. - Pipridae (Manakins).
Family 4. - Cotingidae (Chatterers).
Family 5. - Phytotomidae (Plant-cutters).
Family 6. - Philepittidae (Velvet-thrushes).
Family 7. - Pittidae (Ant-thrushes).
Family 8. - Xeniscidae (Bush-wrens).
Section C. - Tracheophonae.
Family 1. - Dendrocolaptidae (Spine-Tails).
Family 2. - Formicariidae (Ant-birds).
Family 3. - Pteroptochidae (Tapacolas).
Section d. - Passeres abnonnales.
Family 1. - Atrichiidae (Scrub-birds).