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 (c/. 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 {Bep. Brit. Assoc. 1862, pp. 

 422, 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." It must be 

 admitted that birds in confinement sufter 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 aff'ected. 



It 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 "wdth 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 members. These 

 " rushes," as our light-keepers call them, have been so often recorded 

 and described that little need here be said concerning them ; and 



^ 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 oue who has 

 studied the habits of the common House-Fly. 



^ Why Birds or Insects fly at times to a light has never been explained. Birds 

 are often indiff'erent to it, as Mr. G. H. Mackay remarks {Auk, 1891, pp. 340-343). 



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