226 
THE CABINET OF NATURAL HISTORY, 
for the breeding and rearing their young, are the predo- 
minant causes of the migration of birds. The periods 
for these excursions are observed with the most astonish- 
ing order and punctuality; which has been illustrated as 
well as immortalized by Pope, in the following beautiful 
lines: — 
Who taught the nations of the field and flood 
To shun their poison and to choose their food 1 
Prescient, the tides or tempests to withstand, 
Build on the wave, or arch beneath the sand 1 
Who bid the stork, Columbus-like, explore 
Heav’ns not his own, and worlds unknown before 1 
Who calls the council, states the certain day, 
Who forms the phalanx, and who points the way ? 
See then the acting and comparing powers — 
One in their nature, which are two in ours ; 
And reason raise o’er instinct as you can, 
In this ’tis God directs, in that ’tis man. 
The secrecy of the departure of birds, as well as the 
suddenness of their re-appearance, have involved the sub- 
ject of migration in great obscurity. Accustomed to mea- 
sure distances by the speed of those animals with which 
we are well acquainted, we are apt to overlook the supe- 
rior velocity with which birds are carried forward in the 
air, and the ease with which the generality of them con- 
tinue their exertions for a much longer time than the 
strongest quadrupeds are able to effect. Suppose a bird 
to fly half a mile a minute* for twenty-four hours, in that 
space of time it will have gone over an extent of seven 
hundred miles, which is sufficient to account for almost 
the longest migration; and, if aided with favourable cur- 
rents of air, which, when in their highest flights, from 
the appearance of the atmosphere, the clouds, direction 
of the winds, and other causes, they can apply by that 
instinctive knowledge which regulates their movements, 
the journey may be still more speedily performed. Hence 
I can very easily conceive it possible for strong-winged 
birds, like swallows, to reach vast distances across the 
ocean, as well as many others, (the cuckoo for instance,) 
whose powers of flight are very great, and yet inferior to 
those of the swallow. But, in regard to the corn crake, 
the case is different; and the only way in which I can 
think it possible for these birds to cross the sea, is by sup- 
posing that instinct directs them to the straits and narrow- 
est parts, which, with the advantage of a strong current 
of air, they may be able to cross. 
* And birds in general fly much faster. The flight of a crow is at 
least equal to half a mile a minute, or perhaps much more, what then 
must be the speed of that variety of the swallow, distinguished by the 
name of the swift, which I consider as possessing greater speed than 
any other animal in the known world. 
The structure of birds is most wisely and curiously con- 
trived to assist their aerial motion; in every part of their 
form they are active and buoyant, moulded for lightness, 
and shaped for celerity. The bones, according to the ob- 
servations of the late celebrated anatomist, Mr. John Hunter, 
are hollow and contain air, which he imagined might be in- 
tended to assist theanimal in the act of flying, by increasing 
its bulk and strength, without adding to its weight. The in- 
ternal structure of birds is no less wisely adapted. The 
lungs are placed close to the backbone and ribs; the air, 
entering into them by a canal from the windpipe, passes 
through, and is conveyed into a number of membranous 
cells, which lie upon the sides of the pericardium, and 
communicate with those of the sternum. In some birds, 
these cells are continued down the wings, and extend 
even to the pinions, thigh bones, and other parts of the 
body, which can be filled and distended with air at the 
pleasure of the animal. It seems to be evident that this 
general diffusion of air through the bodies of birds is of 
infinite use in assisting respiration in the rapidity of their 
flights. Were it possible for a man to move with the 
swiftness of a swallow, the actual resistance of the air, as 
he is not provided with internal reservoirs similar to those 
of birds, would soon suffocate him. 
The plumage of the bird is admirably adapted to pro- 
tect it from the inclemency of the atmosphere through 
which it passes. The quills of its feathers are firm, yet 
very light; and by the firmness of them it is enabled to 
cleave the air with proper force; while, by their lightness, 
it elevates itself at pleasure. The feathers are placed ge- 
nerally according to their length and strength; so that in 
flight the longest and strongest feathers have the greatest 
share of duty. Nevertheless, the feathers of the bird 
would perpetually imbibe the moisture of the atmos- 
phere, and in every impetuous shower would absorb so 
much wet, as almost, if not wholly, to impede its flight, 
had not the wise economy of nature obviated this by a 
most effectual expedient. The animal is furnished with 
a gland at the extremity of its body, containing a quan- 
tity of unctuous matter, which can be pressed out with 
its bill, and with which it lubricates and anoints its fea- 
thers at pleasure. However, as birds that share, as it 
were, the habitations of man, and live under cover, 
require a more slender supply of this fluid, they are not 
provided with so large a stock as those that rove and re- 
side in the open elements. On this account, therefore, 
domestic poultry are soon affected by wet, a circum- 
stance too well known to need further illustration in this 
place. 
[London Sportsman’s Cabinet . 
