5-t MB. J. H. GURNEY ON ORNITHOLOGICAL MIGRATIONS. 
of migration is past. Probably also they have an instinctive 
knowledge that they must go south in autumn for their food supply; 
but they cannot really have much idea of the direction in which 
to go when they are out at sea. Practical!}'’ it comes to this, 
that all birds in crossing seas must fly against the wind, 
crossing, it may be, some great expanse from one side to the 
other, — with a tendency to go south, and therefore pursuing coast- 
lines -when the wind allows them, in that direction. 
With this proviso, there are many ornithologists who would 
acknowledge themselves believers in the theory of migration by 
sight : but the whole subject is beset with difficulty. 
If in September or October the wind suddenly changes, and 
blows hard from the hi. or ISl. W., a rapid retrograde movement of 
shorebirds at once takes place on the cast coast. I have noticed 
this on three or four occasions, though it seems to have entirely 
escaped the attention of several observers. Of course it is followed 
by a retunr movement, perhaps made at night, and so not noticed. 
The great factors in producing migration are food and the 
direction of the vrind. Birds migrate for -want of food in the 
autumn, and in spring they migrate to a latitude wdiere suitable 
food will be found for their young ones. They move on fast or slow 
according to the varying direction of the wind ; for they always have 
to move against it, if there is any ; though it may bo granted that 
birds, particularly small ones, prefer a day when there is no wind 
at all for migration. 
The state of the temperature has probably nothing whatever to 
do with these movements. Birds can bear a great amount of heat 
or cold with perfect equanimity ; and it is a great mistake to 
suppose that they leave northern regions to get away from the cold, 
against which their feathers, which can be contracted at will into 
a tight-fitting covering, or expanded for fiight and preening, are an 
ample protection. 
The following is an attempt to draw up, for the information of 
ornithologists who are interested in migration, a comparative table 
of all the instances of a specially marked simultaneous migration of 
the same species of bird to the east coast of England, and to 
Heligoland, that little island in the North Sea, so famous for the 
extraordinary number of rare birds ■which have occurred upon it, 
