60 THE BIRDS OF HELIGOLAND 



experiment in which the capacity of birds for living in extremely 

 elevated regions of the atmosphere was submitted to a direct test. 

 This experiment was made by Glaisher and Coxwell, with some 

 pigeons which they took with them on their aerial voyage in 

 England in September 1862. The first of these pigeons was put out 

 after the balloon had reached a height of 16,000. feet ; it spread its 

 wings and appeared to sink, while the balloon was rising with a 

 velocity of 1000 feet per minute. It is probable that it may have 

 soared with calmly outspread wings. The second was put out 

 at a height of 21,000 feet, and wheeled about in powerful flight 

 apparently in a downward direction. The third, put out at a height 

 of about 25,000 feet, dropped like a stone. The balloon reached a 

 height of from 36,000 to 37,000 feet. While it was descending, at 

 the rate of 2000 feet per minute, the fourth pigeon was put out at 

 a height of 21,000 feet. Flying in circles, it followed the balloon 

 as the latter rapidly descended, and perched on the top of it. Of 

 the two remaining pigeons one was found dead at the end of the 

 expedition ; the other, a carrier-pigeon, a quarter of an hour after, 

 flew with fairly powerful motions of the wings to the place whence 

 the balloon had started, whither, two days later, another of the 

 pigeons which had been put out also returned. If, in these experi- 

 ments, captured wild pigeons could have been employed instead of 

 tame ones, the results would undoubtedly have been entirely 

 different. In the first place, it is impossible that tame poultry, not 

 excluding even the best carrier-pigeons, should yield results in regard 

 to flight which might furnish even an approximate standard of 

 comparison in regard to what wild birds might achieve in this direc- 

 tion ; secondly, in experiments of this kind, many different circum- 

 stances come into consideration which were probably not taken 

 note of in the present instance. Thus, all birds obtained directly 

 during the migration have not the least remnants of food in their 

 stomachs : a few small grains of quartz are all that one finds. This is 

 observed not merely in the case of such individuals as may have 

 digested, in the course of a long migration flight, the food taken 

 shortly before their departure ; but also in the case of all such as 

 are captured during the early evening hours of the autumn migra- 

 tion, and therefore probably after a very short flight, as well as in the 

 case of such as are obtained early in the morning during the spring 

 migration, after a flight extending through the night. It appears, 

 therefore, beyond all doubt, that birds do not start on their voyage 

 until the process of digestion is completed, as, for instance, is the 

 case with the small Warblers and Thrushes which in Heligoland 

 start on their migration in May, an hour after sunset or later. 

 A full stomach in itself produces in every creature a disinclination 



