564 MIGRATION 
It has been objected that owing to the extreme cold known to 
prevail at great elevations birds would be unable to travel or even 
exist at some of the heights suggested; but on this point we have 
not yet enough information, any more than as regards the limit 
of cold that birds can endure, which (cf. page 553, note 2) is 
much lower than most people imagine.! The mere exertion of flight 
would certainly for a time keep the body warm, and it is beyond all 
doubt, as Mr. Glaisher’s observations (Rep. Brit. Assoc. 1862, pp. 
429, 423) shew, that the older estimate as to temperature regularly 
decreasing by a degree of Fahrenheit in about every 300 feet of ele- 
vation must be given up, that the decrease is by no means constant, 
and moreover that the atmosphere is traversed by currents of various 
degrees of heat, so that it would often be in the power of any bird 
by raising or lowering its flight to avoid the most chilly stratum.” 
Then again, as every one knows, feathers and down form the best 
non-conducting clothing that exists, and so clad almost every part 
of the bird is not ‘servile to all the skyey influences.” Jt must be 
admitted that birds in confinement suffer in their feet and legs from 
frost, but.it is not known that birds at liberty, and especially in the 
act of taking violent exercise, are so affected. 
Tt has long been remarked that on clear and bright nights birds 
are rarely heard passing over head, while on nights that are over- 
cast, misty and dark, especially if slight rain be falling, flocks may 
often be heard almost continuously. It is in such weather that birds 
while migrating are most vociferous, doubtless with the result that 
thereby the company of fellow-travellers is kept together, and in 
such weather that they fly to and often dash themselves against the 
glasses of light-houses,® occasionally in astonishing numbers. These 
“rushes,” as our light-keepers call them, have been so often recorded 
and described that little need here be said concerning them; and 
1 It is unfortunate that no definite deductions can be made from the be- 
haviour of birds taken up in balloons. Biot and Gay-Lussac, in their celebrated 
ascent of the 24th August 1804, at the height of 11,000 feet, or rather above 2 
miles, liberated a Greenfinch and a Pigeon, each of which after a few turns disap- 
peared downwards through the clouds (Journ. de Phys. lix. p. 318). A similar 
experience was that of Mr. Glaisher in his still loftier ascent of 5th Sept. 1862. 
A Pigeon thrown out at the height of 3 miles ‘‘extended its wings and 
dropped as a piece of paper.” A second at 4 miles “ flew vigorously round 
and round.” A third between 4 and 5 miles ‘fell downwards as a stone.” 
A fourth at 4 miles ‘‘flew in a circle” and then alighted on the balloon 
(Rep. Brit. Assoc. 1862, pp. 385, 386; see also Nature, xix. p. 434, 13 
March, 1879). 
* The quickness with which animals lower in the scale than Birds will avail 
themselves of variation of temperature may be appreciated by any one who has 
studied the habits of the common House-Fly. 
3 Why Birds or Insects fly at times to a light has never been explained. Birds 
are often indifferent to it, as Mr. G. H. Mackay remarks (4wk, 1891, pp. 340-343). 
