Because of the surprise, how migrating birds could support themselves so long on wing, as to accomplish their journeys, and at the same time live without food during their voyage. These difficulties, however, vanish altogether if we attend to the rapidity of the flight of birds. Hawks and many other birds probably fly at the rate of 150 miles an hour: an eider-duck at 90 miles an hour : Sir George Cayley computes the common crow to fly at nearly 25 miles an hour; and Spallanzani found that of the swallow about 92 miles, while he conjectures the rapidity of the swift to be nearly three times greater. A falconwhich belonged to HenryIV of France, escaped from Fontainbleau, and in twenty-four hours afterwards was found at Malta, a distance computed to be no less than 1530 miles; a velocity nearly equal to 57 miles an hour, supposing the falcon to have been unceasingly on the wing. But, as such birds never fly by night, and allowing the day to be at the longest, his flight was perhaps equal to 75 miles an hour. If we even restrict the migratory flight of birds to 50 miles an hour, how easily can they perform their most extensive migrations! Fair winds may perhaps aid them at the rate of 30 or 40 miles an hour; nay with three times greater rapidity. - Fleming.
The migrations of the feathered tribes have been the object of popular observation, since the days of the prophet Jeremiah: " For the stork in the heaven knoweth her appointed times; and the turtle and the crane, and the swallow, observe the time of their coming." (ch. viii. v. 7.)
Because they arrive in this country in the spring, and depart from it in the winter.
Because they arrive in autumn, and depart in spring.
Because all those species in which it is observed, move from the Pole towards the Equator, in search of the temperature congenial to their constitutions, and which the winter of the district of their summer residence could not afford, - Fleming.
Because all the species recede with the increasing temperature of the high latitudes of the Equator, and approach towards the Pole. - Fleming.
Because they are unable sufficiently to provide against the vicissitudes of the seasons, by varying the quantity and colour of their dress; but are thus protected by shifting their quarters, so as to live throughout the whole year in a temperature congenial to their constitutions. - Fleming.
Because, in their annual migrations, birds are occasionally overtaken by storms of contrary wind, and carried far from their usual course.
Mr. White, however, in his Natural History of Sel-borne, says, "It does not appear to me that much stress may be laid on the difficulty and hazard that birds must run in their migrations, by reason of vast oceans, cross-winds, etc.; because, if we reflect, a bird may travel from England to the Equator without launching out or exposing itself to boundless seas - and that by crossing the water at Dover, and again at Gibraltar. And I with the more confidence advance this obvious remark, because my brother has always found that some of his birds, particularly the swallow kind, are very sparing of their pains in crossing the Mediterranean ; for, when arrived at Gibraltar, they do not Ranged in figure, wedge their way, and set forth
Their airy caravan high overseas
Flying, and over lands with mutual wing
Easing their flight; - Milton.
but scout and hurry along in little detached parties of six or seven in a company; and sweeping low, just over the surface of the land and water, direct their course to the opposite continent, at the narrowest passage they can find. They usually slope across the bay to the south west, and so pass over opposite to Tangier, which, it seems, is the narrowest space."