14 THE SCOTTISH NATURALIST 
example of nest-adaptation to a new environment. Our 
Swallow, originally without doubt a cave-dweller, and 
therefore, before the arrival of man, mainly confined to 
the coast and to inland cliffs, has colonised our buildings, 
and thereby been enabled enormously to extend both in 
range and numbers. But this modification in habit is 
‘more apparent than real. To the Swallow a shed or other 
building is merely a species of cave, and in utilising it 
for its nest it can hardly be said to have made any true 
change in habit. A similar remark applies to such a case 
as that of the Swift, another bird which has made use of 
man’s buildings, but only, as we believe, because of their 
close approximation in character to the ancestral rock 
or cliff. On the other hand, there are not a few instances 
of radical modification of feeding-habit, and the most 
frequently quoted example, and one of the most striking, 
is the New Zealand Kea, which, originally a fruit-eater, 
has within a few generations taken to a flesh diet. 
In view, therefore, of its essential importance, it seems 
most convenient to take the nesting-ground as the basis 
of an ecological classification. This limits a faunal discussion 
to those species on the British list which breed within the 
limits of the area. I believe such a definition to be the only 
scientific one: a bird is British only when it is literally a 
native, and such a species as the Fieldfare on the one hand, 
or the Yellow-browed Warbler on the other, no more deserve 
inclusion as British species than does the American tourist 
deserve to be regarded as a British citizen. 
A bird-association accordingly, includes that group of 
species found associated together during the nesting-season 
on ground of a certain geological or botanical type; and the 
wider divisions in which botanists class their plant forma- 
tions, are in practice found to mark off corresponding 
bird-associations. Apart from mere convenience in classifica- 
tion, such a grouping is important, since it leads to such 
questions as the inter-relationships of bird and plant 
species. 
The Scottish avifauna might in the first instance be 
classified into the following wider groups of Associations, each 
