294 FACULTIES OF BIRDS. 
a mile a minute, which is less than I have actually 
ascertained them to do over land, ten or twelve 
hours would be sufficient to accomplish the jour- 
ney.” 
The periodical migration of the gannet affords an 
instance of a mixed nature. That bird arrives early 
in spring, and is located in four or five spots along 
the British coasts, of which may be mentioned the 
Bass Rock, Ailsa Craig, and St. Kilda. In autumn 
the gannets leave their breeding-places, and are 
seen ulong the coast of England and in the Chan- 
nel. In mild winters some individuals often re- 
main, and even the whole flock has been known 
to winter in their summer residence. Even when 
they all leave the breeding-places, many individuals 
do not extend their migration beyond the southern 
coasts of England, but where the extreme point of 
the range may be has not yet been ascertained. 
In all these cases, the distribution of food seems 
to be the principal cause of the movements of the 
birds; but in other cases it is clear that the rigour 
of the winter also acts as an exciting cause; yet it 
is doubtful whether cold alone be sufficient to drive 
birds from their northern haunts. Fieldfares and 
redwings, no doubt, leave the northern parts of Eu- 
rope at the end of autumn, because at that period 
the ground begins to be covered with snow, so that 
they are unable any longer to procure food; but 
they merely shift, so as to place themselves on the 
limits of the storm, their object being apparently 
more to obtain the necessary supplies than to evade 
the cold. In mild and open winters they remain 
until late in spring; whereas, after snow has con- 
tinued several weeks on the ground, it is seldom 
that any are to be seen. As to swallows, it is evi- 
dent that the same cause operates most powerfully 
on them, because, as we have seen, they are capa- 
ble of bearing as much cold as other small birds. 
How far the migrations of birds may extend, has 
