14 
OENERALIA 
some species, their disappearance and reappearance in other 
regions; they explain the existence of birds which merely migrate 
from hill to valley and vice-versa, as well as of those which 
remain in one place for years, changing their diet according 
to the season, enduring all hardships and not departing even 
in times of want; and, finally, they render intelligible the 
stupendous system of „ migration", a phenomenon closely 
bound up with the change of the seasons, the essential point 
of which is that many birds peculiar to northern regions 
leave home in winter for the South and wander to our temp- 
erate zone, whereas many of our feathered friends desert 
us when the cold weather approaches and retire to far distant 
southern climes. When spring comes, our own breeds return, 
while those who hibernated with us leave for their northern 
homes. 
If we survey all these points, we may get a true if somewhat 
hazy picture of the significance of birds as represented in 
the work which neither Nature nor Man can dispense with 
with impunity, the loss of which cannot be supplied by any 
manner of artificial procedure. 
It is particularly the life of insects and herbs that the labour 
of birds is called upon to regulate. The proportions of this 
labour are well illustrated, in the North, by the millions of 
gulls that live on animal food; in our climate, on the other 
hand, in places where human hands have but slightly or 
not at all disturbed the primeval state of Nature, by the 
cloud-like hosts of starlings and crows which are joined by 
thousands upon thousands of smaller gregarious seed-eating 
birds that prevent the over-growth of weeds. 
Keen-eyed observers will discover that, to do certain forms 
of labour that only birds can accomplish, birds of widely 
diverse structure fly together or rather form organised, sym- 
biotic labourer-bands. In our climate the most remarkable 
