278 
THE LIVING WORLD. 
their eggs, it is an invariable law among every species of bird that their repro¬ 
duction shall be by the deposition of eggs, yet, however, as seen above, all 
species do not incubate. 
HOW BIRDS SAIL THROUGH THE AIR. 
The most striking feature of bird physiology, as showing a marvellous 
creative design that impels our reverence for a wisdom so infinite, is found in 
the skeletal structure and wonderful form, substance and adaptation of the 
feathery covering, by which flight is rendered at once easy and natural. While 
the bones of all other animals now existent are solid, the centre being a cone 
of marrow, in birds there is extreme lightness without the sacrifice of strength, 
by the bones being hollow and braced, as it were, by numerous osseous capil¬ 
lary ducts, through which a circulation of heated air is in constant passage. 
We look with mingled surprise and wonder at the hawk, buzzard, and some 
other species, as they go sliding through the air with only an occasional flap 
of the wing, seemingly sustained by some invisible force, and propelled by an 
agency that appears difficult to understand. If one of these sailing birds be 
taken and submitted to certain experiments, including dissection, it will be 
found that the bones are not only hollow and extremely light, but that they 
are very warm to the touch. 
If the end of a wing be 
lopped off, and the bird’s 
windpipe closed, however 
tightly, it may still be seen 
to breathe with the same 
regularity as before being 
injured. By this experi¬ 
ment we are given to know 
that both heart and lungs 
are directly connected with 
all the bones, that the heart 
action is not confined to 
pumping blood, but that it 
also pumps air through all 
the bony channels and feath¬ 
ers ; this air is heated by the heart and then distributed through every canal, 
however small, in the entire skeleton. We find also that the normal temper¬ 
ature of birds is ten degrees greater than in mammals, and this normal tem¬ 
perature may be increased by at least ten degrees more, at the will of the bird. 
This produces a rarefaction of the air within the bones sufficient to give con ¬ 
siderable buoyancy, to which sustaining effect is added the lifting power whic 1 
resides within the feathers. This force is ill understood, but that it exists i, 
demonstrated by the fact that to divest the body of a bird of its feathers, though 
the wings may be left intact, destroys its power of flight. 
In addition to the knowledge gained by experiments which are occasionally 
cruel, having to be made while the bird is alive, is the striking contrast 
between birds of flight and those destitute of this ability. We know that the 
bones of domestic fowls are thicker and heavier than those of the wild species, 
and investigation has also shown that in such birds as the penguin, ostrich, kiwi, 
« 1 
DIFFERENT POSITIONS ASSUMED BY BIRDS WHEN MIGRATING, a. DUCKS, 
b. EUROPEAN IBIS. C. GEESE, d. CRANES, e. BUZZARDS, f. DIVERS, 
g. OYSTER-FISHERS, h. SAND-PIPERS. 
