BIRD-ASSOCIATIONS IN SCOTLAND 13 
and its accompanying invertebrate fauna, and thus indirectly, 
and in a degree directly, on the underlying geology. There 
are of course other factors, such as climate, inter-rela- 
tions of species and so forth, but these are probably 
in the main of somewhat secondary importance. Climate, 
in so far as it affects the plant-formation, may exert a 
profound influence upon the avifauna, but its direct effect 
is apparently not of great value. That this is so, seems 
to be borne out by the manner in which dominant species 
range throughout many degrees of latitude. 
In dealing with birds, we are met at the outset 
by a difficulty which does not trouble the botanist or the 
entomologist. | Food-supply is equally essential in each 
case, but with birds there arises the additional requisite of 
nesting-ground, and we know that it is not less necessary 
than the former. In many groups, such as Plovers, Larks, 
Pipits, etc, the two essentials are co-existent, the birds 
nesting on their feeding-grounds. In others, such as Crows, 
Swallows, Hawks, sea-fowl, the two are separated, and 
the specific range is absolutely limited by whichever of the 
two, food or nesting-ground, is relatively inadequate. To 
realise how a narrowly defined nesting-habit may affect a 
bird’s very existence, it is only necessary to picture the effect 
on the British avifauna which would be brought about by 
the complete disappearance of our rocky coasts and islands, 
We would at once lose our Auks, nearly all our Gulls, 
Petrels, and many others. They would vanish as British 
species, not because of a failure in food-supply, but by 
reason of a lack of nesting-ground. 
It is probable that birds, as a class, are more conservative 
as regards nesting than as regards food. We know too 
little of the latter subject to dogmatise, but considering 
how persistently a certain nest-type is found running 
through a genus or family otherwise variable, the suggestion 
seems not unreasonable. It almost suggests that the 
evolution of the nest and the nesting-habit have tended to 
lag behind in the development of the race, and it is easy to 
picture how such a tendency may have checked the increase 
or dispersal of a species. The Swallow is often quoted as an 
