16 THE SCOTTISH NATURALIST 
Taking next the mountain-cliff, we reach an association _ 
ofa new aspect, distinguished from those already touched on 
by its relatively unstable nature. A mountain-cliffis subjected 
in an exceptional degree to alteration in profile through 
weathering, and the resulting instability is.not without effect 
on the avifauna, which is likewise unstable. In a botanical 
sense, an association of this character is distinguished by Dr 
Crampton from those of a relatively stable nature by the 
term migratory; and this distinction is worth keeping in 
mind likewise in the case of birds. Later, when we come to 
Woodlands, I shall have occasion to refer again to the 
contrast between stable and migratory associations. The 
more prominent birds associated with the mountain-cliff 
fauna, are: Raven, Peregrine, Kestrel, Buzzard, and Golden 
Eagle. The existence and comparative numbers of each of 
these in a district is dependent not only upon the supply of 
rocks, but upon their structure and manner of weathering. 
Every field-man is aware that the nest requirements vary 
in each species of this list. An Eagle requires a ledge 
of a proportion not by any means to be found on all cliffs. 
The Peregrine seeks a comfortable grassy ledge commanding 
a wide outlook. The Buzzard prefers to rest his nest partly 
upon the roots of a tree growing in a crevice in the rock. 
The Raven regards no site as eligible which is not provided 
with a heavy overhang above the nest, and, it might be 
incidentally added, a nasty climb below. The cliff-factor 
in the distribution of these birds is one which must be 
reckoned with, and it would be a fascinating subject for 
enquiry to follow out how far their comparative numbers and 
range are correlated with it, as compared with the food- 
supply. There are some apparent anomalies to be explained. 
In the sheep-country of central Scotland, for example, as 
contrasted with the deer-forest of the Cairngorms, the Raven 
and Peregrine are relatively numerous, and the Golden Eagle 
relatively scarce. How far is this correlated with food-supply 
or with nesting-ground? A friend of mine, to whom I 
recently put the question, rather took me aback by the 
flippant suggestion that it was less a question of food-supply 
or nesting-ground than one of the psychology of the Highland 
