MIGRATION. 
387 
fine days and bright calm nights are advantageous to the 
journey; head-winds are, as we have before stated, not 
contrary in the sense of being impediments to the bird’s 
advance, but those blowing in the same direction as the 
birds are travelling are so. That such winds (north-east 
and east) render their journey much more laborious is 
proved by the weakly state of those Larks caught, and 
the poor condition of those killed, which have had to 
battle with them for several days. Birds of passage 
further prove this by the way in which they are led away 
from their real route by a due south or westerly wind, as 
well as by valleys and river-beds. A head-wind is a real 
necessity in the case of long migratory flights. Both 
foggy and boisterous weather in some measure alter the 
mode in which the “passage” is conducted. Thus on 
dark nights all cautious, and especially shy, birds, fly 
close to the ground, whereas in the daytime, or on clear 
nights, they often fly so high as to be almost invisible to 
the naked eye, their presence being detected solely by 
their call-notes. They do this, doubtless, from prudential 
motives. I have learnt this when shooting Cranes at night. 
If bad weather lasts for several days most birds of 
passage remain quite quiet, occupying themselves wholly 
and totally with feeding. It seems as though they were 
laying up store against the days of travel they have in 
prospect, when unwonted energy and strength will be 
called in requisition, for they cease not to feed the live¬ 
long day, laying on a goodly coating of fat, which, 
however, under the influence of extraordinary exertion, 
disappears as rapidly as it accumulated. 
We have now described the mode of migration pursued 
by birds who make their journey wholly on the wing. It 
is not only highly probable, but in part proven, that 
