50 THE SCOTTISH NATURALIST 
40 colonies, each living and roosting separately, we have 
only one general roosting-place, to which this whole 8000 
pairs of birds resort, together with their young. While it 
might be said that each of the nesting-colonies drains an 
area of 3 sq. miles, the solitary winter-rookery drains an 
area of 120 sq. miles, and this without taking into considera- 
tion that a large section of Midlothian is likewise drawn 
upon, and probably portions of other adjoining counties as 
well. This example is sufficient, I think, to indicate that 
even so social a species as the Rook is no true exception to 
the general rule. 
But when we come to sea-birds, it is hardly an exaggeration 
to say that the exact contrary is the case, and the various 
members, while never very solitary of habit, reach their most 
social phase during the breeding season. I believe the 
explanation to be primarily one of nesting-ground. Whereas 
in land-birds the supply of nesting-ground is relatively 
adequate to the food-supply, in sea-birds there is, as it were, 
a superabundance of food as compared with the available 
nesting-ground. I suggest that the colonial habit in sea-birds 
is a result rather of the force of circumstances—z.¢., a combina- 
tion of the two factors mentioned—than ofa natural sociability 
of temperament. The relative abundance of food tends to 
an increase of population, constantly and severely held in 
check by the lack of nest-sites. An observer of a great 
Guillemot or Gannet colony cannot but be struck by the 
absence of friendly feelings, the constant struggling and 
bickering which takes place on the crowded ledges, and 
it is difficult to doubt that lack of alternatives is the essential 
cause of the close packing of the birds. 
The avifauna of the sea-cliff is absolutely correlated with 
the geological formation, upon the constitution, lie of the 
strata, and manner of weathering. Some cliffs, such as 
certain types of Old Red Sandstone, in many cases weather 
in great perpendicular slices, leaving a clean wall of rock 
with few ledges or crevices, and therefore not adapted for 
colonisation by birds. Where we have a cliff with a 
horizontal or gently inclined plane, and especially where 
the strata are of varying powers of resistance to denudation, 
