THE ANATOMY OF BIRDS.— OOLOGY. 227 
to be incubated and hatched simultaneously. It is not so unusual among American cuckoos 
as generally supposed. The degree of development to which birds attain in the egg has been 
already discussed (p. 88). They break the shell by pecking at it, and struggling ; for the 
former operation the bill is often tempered at the tip by a hard knob which is afterward ab- 
sorbed. The necessity of providing a receptacle for eggs, in which they may be incubated, 
results in nidification or nest-building ; and the extraordinary taste and ability many birds dis- 
play in this matter, as well as the wide range of their habitudes, furnishes one of the most 
delightful departments of ornithology, called caliology (Gr. xadtd, kalia, a bird’s nest; see 
p. 54, note). Many birds burrow in the ground; others in trees; the most beautiful and 
elaborate nests are furnished by various members of the Oscines, the weaver-birds of Africa 
(Ploceide) probably taking the lead. The male sometimes constructs his own “nest” apart 
from that in which the female ineubates. ‘Certain conirostral Cantores still practise in the 
undisturbed wilds of Australia the formation of marriage-bowers distinct from the later-formed 
nesting-place. The satin bower-bird (Ptilonorhynchus holosericeus), and the pink-necked 
bower-bird (Chlamydodera maculata), are remarkable for their construction on the ground of 
avenues, over-arched by long twigs or grass-stems, the entry and exit of which are adorned by 
pearly shells, bright-colored feathers, bleached bones, and other decorative materials, which are 
brought in profusion by the male, and variously arranged to attract, as it would seem, the 
female by the show of a handsome establishment” (Owen). The extraordinary nests of the 
Crotophaga, used in common by a colony of the birds, are noted at p. 471. ‘‘ Edible birds’- 
nests,” constructed by swifts of the genus Collocalia, consist chiefly of inspissated saliva. 
Perhaps the most remarkable of all the receptacles of eggs is that which the penguin makes of 
its own body, the egg being carried in a sort of pouch formed by the integument of the belly, 
something like that of a marsupial mammal. 
. 
§ 5. DIRECTIONS FOR USING THE ARTIFICIAL KEYS. 
These “Keys”. differ from natural analyses in being wholly arbitrary and artificial. 
They are an attempt to take the student by a ‘‘short cut” to the name and position in the orni- 
thological system of any specimen of a North American bird he may have in hand and desire to 
identify. The plan has been much used in Botany, though seldom if ever employed for a 
whole Fauna, before the original edition of this work. It will serve a good purpose, rightly 
used; but it’ must be remembered there is no ‘‘royal road to learning”; nobody can be 
smuggled into sound erudition, either. Nor must too much be expected of me here; I can 
take the student nowhere until he has learned the difference between the head and the tail of 
a bird, at any rate. That is what the preceding pages undertake to teach; but, until such 
technicalities have been mastered, progress in ornithology is out of the question. 
The original ‘‘ Key to the Genera” proved scarcely so satisfactory as I hoped it would be. 
It undertook too much, to conduct the student at once down to the intricacies of the very 
many modern genera, not all of which can by any possibility be characterized intelligibly in 
a line of type. I have probably simplified and expedited matters by preparing on the same 
plan Keys to the Orders and Sub-orders, and to the Families. ‘Then in the body of the 
work, under each head, further analyses are given when such seems to be required, — of 
families under their orders or sub-orders, of genera under their families, and of species under 
their genera. These ulterior analyses are for the most part rather natural than artificial, 
though I never hesitate to seize upon any character that may furnish the desired clue to ideuti- 
fication. 
The artificial Keys immediately following will take the student to the families, with refer- 
ence to the page of the work where such groups come; on turning to which, further analyses 
