Chapter III: Introduction

The class of Birds (_Aves_), as represented in the present age of the world, is composed of very many species, closely related among themselves and distinguished by numerous characters common to all. For the purposes of the present work it is hardly necessary to attempt the definition of what constitutes a bird, the veriest tyro being able to decide as to the fact in regard to any North American animal. Nevertheless, for the sake of greater completeness, we may say that, compared with other classes,[1] Birds are abranchiate vertebrates, with a brain filling the cranial cavity, the cerebral portion of which is moderately well developed, the corpora striata connected by a small anterior commissure (no corpus callosum developed), prosencephalic hemispheres large, the optic lobes lateral, the cerebellum transversely multifissured; the lungs and heart not separated by a diaphragm from the abdominal viscera; aortic arch single (the right only being developed); blood, with nucleated red corpuscles, undergoing a complete circulation, being received and transmitted by the right half of the quadrilocular heart to the lungs for aeration (and thus warmed), and afterwards returned by the other half through the system (there being no communication between the arterial and venous portions); skull with a single median convex condyle, chiefly on the basi-occipital (with the sutures for the most part early obliterated); the lower jaw with its rami ossifying from several points, connected with the skull by the intervention of a quadrate bone (homologous with the malleus); pelvis with ilia prolonged in front of the acetabulum, ischia and pubes nearly parallel with each other, and the ischia usually separated: anterior and posterior members much differentiated; the former modified for flight, with the humerus nearly parallel with the axis of the body and concealed in the muscles, the radius and ulna distinct, with two persistent carpal bones, and two to four digits; the legs with the bones peculiarly combined, (1) the proximal tarsal bones coalescing with the adjoining tibia, and (2) the distal tarsal coalescing with three (second, third, and fourth) metatarsals (the first metatarsal being free), and forming the so-called tarsometatarsus; dermal appendages developed as feathers: oviparous, the eggs being fertilized within the body, excluded with an oval, calcareous shell, and hatched at a temperature of about 104° F. (generally by the incubation upon them of the mother).[2]

Such are some of the features common to all the existing species of birds.[3] Many others might be enumerated, but only those are given which contrast with the characteristics of the mammals on the one hand and those of the reptiles on the other. The inferior vertebrates are distinguished by so many salient characters and are so widely separated from the higher that they need not be compared with the present class.

Although birds are of course readily recognizable by the observer, and are definable at once, existing under present conditions, as warm-blooded vertebrates, with the anterior members primitively adapted for flight,—they are sometimes abortive,—and covered with feathers, such characteristics do not suffice to enable us to appreciate the relations of the class. The characteristics have been given more fully in order to permit a comparison between the members of the class and those of the mammals and reptiles. The class is without exception the most homogeneous in the animal kingdom; and among the living forms less differences are observable than between the representatives of many natural orders among other classes. But still the differences between them and the other existing forms are sufficient, perhaps, to authorize the distinction of the group as a class, and such rank has always been allowed excepting by one recent naturalist.

But if we further compare the characters of the class, it becomes evident that those shared in common with the reptiles are much more numerous than those shared with the mammals. In this respect the views of naturalists have changed within recent years. Formerly the two characteristics shared with the mammals—the quadrilocular heart and warm blood—were deemed evidences of the close affinity of the two groups, and they were consequently combined as a section of the vertebrates, under the name of Warm-blooded Vertebrates. But recently the tendency has been, and very justly, to consider the birds and reptiles as members of a common group, separated on the one hand from the mammals and on the other from the batrachians; and to this combination of birds and reptiles has been given the name _Sauropsida_.

As already indicated, the range of variation within this class is extremely limited; and if our views respecting the taxonomic value of the subdivisions are influenced by this condition of things, we are obliged to deny to the groups of living birds the right which has generally been conceded of ranking as orders.

The greatest distinctions existing among the living members of the class are exhibited on the one hand by the Ostriches and Kiwis and the related forms, and on the other by all the remaining birds.

These contrasted groups have been regarded by Professor Huxley as of ordinal value; but the differences are so slight, in comparison with those which have received ordinal distinction in other classes, that the expediency of giving them that value is extremely doubtful; and they can be combined into one order, which may appropriately bear the name of _Eurhipidura_.

An objection has been urged to this depreciation of the value of the subdivisions of the class, on the ground that the peculiar adaptation for flight, which is the prominent characteristic of birds, is incapable of being combined with a wider range of form. This is, at most, an explanation of the cause of the slight range of variation, and should not therefore affect the exposition of the _fact_ (thereby admitted) in a classification based on morphological characteristics. But it must also be borne in mind that flight is by no means incompatible with extreme modifications, not only of the organs of flight, but of other parts, as is well exemplified in the case of bats and the extinct pterodactyls.

Nor is the class of birds as now limited confined to the single order of which only we have living representatives. In fossil forms we have, if the differences assumed be confirmed, types of two distinct orders, one being represented by the genus _Archæopteryx_ and another by the genera _Ichthyornis_ and _Apatornis_ of Marsh. The first has been named _Saururæ_ by Hæckel; the second _Ichthyornithides_ by Marsh.

Compelled thus to question the existence of any groups of ordinal value among recent birds, we proceed now to examine the grounds upon which natural subdivisions should be based. The prominent features in the classification of the class until recently have been the divisions into groups distinguished by their adaptation for different modes of life; that is, whether aerial or for progression on land, for wading or for swimming; or, again, into Land and Water Birds. Such groups have a certain value as simply artificial combinations, but we must not be considered as thereby committing ourselves to such a system as a natural one.

The time has scarcely arrived to justify any system of classification hitherto proposed, and we can only have a sure foundation after an exhaustive study of the osteology, as well as the neurology and splanchnology, of the various members. Enough, however, has already been done to convince us that the subdivision of the class into Land and Water Birds does not express the true relations of the members embraced under those heads. Enough has also been adduced to enable us to group many forms into families and somewhat more comprehensive groups, definable by osteological and other characters. Such are the Charadrimorphæ, Cecomorphæ, Alectoromorphæ, Pteroclomorphæ, Peristeromorphæ, Coracomorphæ, Cypselomorphæ, Celeomorphæ, Aëtomorphæ, and several others. But it is very doubtful whether the true clew to the affinities of the groups thus determined has been found in the relations of the vomer and contiguous bones. The families, too, have been probably, in a number of cases, especially for the passerine birds, too much circumscribed. The progress of systematic ornithology, however, has been so rapid within the last few years, that we may be allowed to hope that in a second edition of this work the means may be furnished for a strictly scientific classification and sequence of the families. (T. N. G.)

A primary division of recent birds may be made by separation of the (_a_) _Ratitæ_, or struthious birds and their allies,—in which the sternum has no keel, is developed from lateral paired centres of ossification, and in which there are numerous other structural peculiarities of high taxonomic import,—from the (_b_) _Carinatæ_, including all remaining birds of the present geologic epoch. Other primary divisions, such as that into _Altrices_ and _Præcoces_ of Bonaparte, or the corresponding yet somewhat modified and improved _Psilopaedes_ and _Ptilopaedes_ of Sundevall, are open to the serious objections that they ignore the profound distinctions between struthious and other birds, require too numerous exceptions, cannot be primarily determined by examination of adult specimens, and are based upon physiological considerations not necessarily co-ordinate with actual physical structure.

In the following scheme, without attempting to indicate positive taxonomic rank, and without committing myself finally, I present a number of higher groups into which Carinate birds may be divided, capable of approximately exact definition, and apparently of approximately equivalent taxonomic value. Points of the arrangement are freely drawn from the writings of various authors, as will be perceived by those competent to judge without special references. I am particularly indebted, however, to the late admirable and highly important work of Professor Sundevall,[4] from which very many characters are directly borrowed. The arrangement, in effect, is a modification of that adopted by me in the “Key to North American Birds,” upon considerations similar to those herewith implied. The main points of difference are non-recognition of three leading groups of aerial, terrestrial, and natatorial birds,—groups without morphological basis, resting simply upon teleological modification; a general depreciation of the taxonomic value of the several groups, conformably with the considerations presented in the preceding pages of this work; abolishing of the group _Grallatores_; and recognition of a primary group _Sphenisci_.[5]

A. PASSERES.[6] Hallux invariably present, completely
incumbent, separately movable by specialization of the _flexor
hallucis longus_, with enlarged base and its claw larger than
that of the middle digit. Neither second nor fourth toe
versatile; joints of toes always 2, 3, 4, 5, from first to
fourth. Wing-coverts comparatively short and few; with the
exception of the least coverts upon the _plica alaris_,
arranged in only two series, the greater of which does not
reach beyond the middle of the secondary remiges.[7] Rectrices
twelve (with rare anomalous exceptions). Musical apparatus
present in greater or less development and complexity. Palate
ægithognathous. Sternum of one particular mould,
single-notched. Carotid single (sinistra). Nature highly
altricial and psilopædic.

a. Oscines.[8] Sides of the tarsus covered in most or all of
their extent with two undivided horny plates meeting behind
in a sharp ridge (except in _Alaudidæ_; one of the plates
imperfectly divided in a few other forms). Musical apparatus
highly developed, consisting of several distinct pairs of
syringeal muscles. Primaries nine only, or ten with the first
frequently spurious, rarely over two thirds the length of the
longest, never equalling the longest.

b. Clamatores.[9] Sides of the tarsus covered with divided
plates or scales variously arranged, its hinder edge blunt.
Musical apparatus weak and imperfect, of few or incompletely
distinguished syringeal muscles (as far as known). Primaries
ten with rare exceptions, the first usually equalling or
exceeding the rest.

B. PICARIÆ.[10] Hallux inconsiderable, weak or wanting, not
always incumbent, not separately movable by distinction of a
special muscle, its claw not longer than that of the middle toe
unless of exceptional shape (e. g. _Centropus_). Second or
fourth toe frequently versatile; third and fourth frequently
with decreased number of joints. Wing-coverts for the most part
larger and in more numerous series than in _Passeres_, the
greater series reaching beyond the middle of the secondary
quills (except in many _Pici_ and some others). Rectrices
commonly ten (eight to twelve). Primaries always ten, the first
only exceptionally short (as in _Pici_). Musical apparatus
wanting, or consisting of a muscular mass, or of not more than
three pairs of syringeal muscles. Palate desmognathous or
ægithognathous. Sternum of non-passerine character, its
posterior border entire or doubly notched or fenestrate.
Carotid single or double. Nature completely altricial, but
young sometimes hatched with down[11] (e. g. _Caprimulgidæ_).

a. Cypseli. Palate ægithognathous. Wings lengthened in their
terminal portions, abbreviated basally, with the first
primary not reduced. Tail of ten rectrices. Bill fissirostral
or tenuirostral. Feet never zygodactyle nor syndactyle,
small, weak, scarcely fitted for locomotion; hallux often
elevated or lateral or reversed; front toes usually webbed at
base, or with abnormal ratio of phalanges in length and
number, or both. Sternum deep-keeled, usually entire or else
doubly notched or perforate. Syringeal muscles not more than
one pair.

b. Cuculi. Palate desmognathous. Wings not peculiar in
brevity of proximal or length of distal portions, and with
first primary not reduced. Tail of eight to twelve rectrices.
Bill of indeterminate form, never cered; tongue not
extensile. Feet variously modified by versatility or
reversion of either first, second, or fourth toes, or by
cohesion for a great distance of third and fourth, or by
absence or rudimentary condition of first or second; often
highly scansorial, rarely ambulatorial. Syringeal muscles two
pairs at most.

c. Pici. Palate “exhibiting a simplification and degradation
of the ægithognathous structure” (Huxley); wings bearing out
this passerine affinity in the common reduction of the first
primary and the restriction of the greater coverts. Tail of
ten perfect rectrices and usually a supplementary pair.
Rostrum hard, straight, narrow, subequal to head, with
commonly extensile and vermiform but not furcate tongue. Feet
highly scansorial. Fourth toe permanently reversed; basal
phalanges of toes abbreviated. Sternum doubly notched.
Salivary glands highly developed. Hyoidean apparatus peculiar.

C. PSITTACI. Bill enormously thick, short, high, much arched
from the base, the upper mandible strongly hooked at the end,
cered at base, and freely movable by complete articulation with
the forehead, the under mandible with short, broad, truncate
symphysis. Feet permanently zygodactyle by reversion of the
fourth toe, which articulates by a double facet. Tarsi
reticulate. Syrinx peculiarly constructed of three pairs of
intrinsic muscles. Tongue short, thick, fleshy. Sternum entire
or fenestrate. Clavicles weak, defective, or wanting. Orbit
more or less completed by approach or union of postorbital
process and lachrymal. Altricial; psilopædic.