MIGRATION. 297 
pable of carrying them over a very large space in a 
short time. ‘The flight of birds generally may be 
estimated at from fifty to one hundred and twenty 
miles an hour; and if we take the mean of this, we 
shall find it sufficient to enable the migratory birds 
to perform the most extended journeys. The won- 
der is not in the flight itself, but in the impulse and 
instinct by which it is commenced and carried on. _ 
Pennant finds no difficulty in accounting for the 
motive of migrations: a defect of food at certain 
seasons, or the want of a secure asylum from the 
persecutions of man during the time of courtship, 
incubation, and nutrition. He considers that most 
of the birds which leave England in spring, to spend 
the summer elsewhere, have been traced to Lap- 
land, a country of lakes, rivers, swamps, and alps, 
covered with thick and gloomy forests, that afford 
shelter during summer to these fowls, which in 
winter disperse over the greater part of Europe. 
In these arctic regions, in consequence of the 
thickness of the woods, the ground remains soft 
and penetrable to the woodcocks and other slen- 
der-billed fowls; and for the web-footed birds the 
water affords innumerable larve of the gnat. The 
days are there long, and the beautiful meteorous 
nights indulge them with every opportunity of 
collecting so minute a food, while mankind is 
very sparingly scattered over those vast northern 
wastes. 
The migration of winter birds of passage doubt- 
less proceeds on the same general law as that which 
regulates the movements of those birds which spend 
the summer in England and leave it in winter. 
Birds which find the temperature and circumstances 
of summer in that country most congenial to their 
wants and habits, retire on the approach of severe 
weather to find something similar in the south; 
while others, which remain there in winter to 
avoid the extreme rigour of that season in the most 
