108 



out having been able to make a start on their journeys." The 

 little aid furnished by the air-sacs to a bird during its migration 

 has already been pointed out, and apparently recognised by Herr 

 Gatke. It is probably true that they may be fiUed or emptied 

 at pleasure, but whether a bird has the power of retaining their 

 contents or keeping them void is quite another matter. The 

 objection that birds might be so detained by stress of weather — • 

 unless they performed their migrations at great speed and at a 

 great height — as to prevent their reaching their breeding grounds 

 at all, is overcome in so simple a manner, i.e., by their migrating 

 in shorter or longer stages as opportunity allowed, that it is 

 rejected by Herr Gatke, apparently for this reason alone. " To 

 withdraw themselves," he further adds, " from the disturbing 

 influences which are apt to prevail in these changeful lower strata 

 birds mount up into the more elevated layers of the atmosphere, 

 in which more uniform conditions prevail, and which are less 

 subject to powerful ineteorological disturbances. In this way, 

 however, they reach elevations at which the resistance of the air 

 is so insignificant as to render possible the astonishing velocity 

 developed during the migration, while this velocity at the same 

 time counteracts any tendency towards sinking, a slight elevation 

 of the anterior margin of the horizontal wing surface being amply 

 sufficient to effect this object." 



All of this, has already been commented upon, but why the 

 author should imagine birds must ascend to these heights to 

 accomplish their migrations, when — in the light of his own 

 assertions — a sluggish flyer, like the Hooded Crow, or an expert 

 one, like the Virginian Plover, can develop respective speeds 

 amounting to 108 and 212 "geographical" miles an hour, at a 

 height of only a few feet above the surface of the land or sea, 

 is most difiicult to surmise. 



