170 NATURAL HISTORY 
above-mentioned, he remarks that the procession 
is swelled by whole troops of eagles and vultures. 
Now it is no wonder that birds residing in Africa 
should retreat before the sun as it advances, and re- 
tire to milder regions, and especially birds of prey, 
whose blood being heated with hot animal food, are. 
more impatient of a sultry climate ; but then I can- 
not help wondering why kites and hawks, and such 
hardy birds as are known to defy all the severity of 
England, and even of Sweden and all north Europe, 
should want to migrate from the south of Europe, 
and be dissatisfied with the winters of Andalusia. 
It does not appear to me that much stress may — 
be laid on the difficulty and hazard that birds must 
run in their migrations, by reason of vast oceans, 
cross winds, &c. ;- because, if we reflect, a bird 
may travel from England to the equator without 
launching out and exposing itself to boundless seas, 
and that by crossing the water at Dover, and again 
at Gibraltar. And I with the more confidence ad- 
vance this obvious remark, because my brother has 
always found that some of his birds, and particu- 
larly the swallow kind, are very sparing of their 
pains in crossing the Mediterranean ; for, when ar- 
rived at Gibraltar, they do not, 
ad ' Ranged i in figure, wedge their way, 
eth * and set forth 
Their airy caravan high over seas 
Flying, and over Jands with mutual wing i 
Easing their flight ;” MILTon. 
but scout and hurry along in little detached parties 
of six or seven in a company ; and sweeping low, 
just over the surface of the land and water, direct 
their course to the opposite continent at the nar- 
rowest passage they can find. They usually slope 
or 
a i 
