nditions must prevail in many situations; but, clearly, the
more binding and plastic the building material, the longer the nest will
withstand the action of the dripping water and the greater chance will
there be of the young being reared in safety. Observe, therefore, how
far-reaching an effect so small a detail as the nature of the mud can
have upon the status of the species in any given locality. Where the
conditions are favourable, there the birds must congregate to breed,
and, like the Guillemot, if each individual exercised dominion over too
large an area, the species as a whole would suffer.

In all these examples, the fact of different individuals being in such
close proximity may afford some protection from enemies both as regards
the egg and the offspring, and in so far as there is a mutual advantage
such assemblages may be spoken of as communities. A community, however,
in the true sense of the word, is a collection of individuals brought
together, not primarily as a result of shortage of breeding ground, but
in consequence of advantages of communal ownership over individual
ownership. A rookery is an example of a true community. Neither shortage
of nesting accommodation nor scarcity of food can account for Rooks
assembling together to breed; for if the different pairs which go to
make up the rookery were to scatter throughout the surrounding
neighbourhood, they would, as a rule, find plenty of trees in which to
build their nests, and plenty of food.

How, then, can the theory apply to a species that breeds under such
conditions? What part can the territory play in furthering the life of
the individual when large numbers of nests are built closely together in
the same tree? There is much evidence to show that mutual protection is
a necessary condition of the Rook's existence; many cases are on record
of rookeries being destroyed by Carrion-Crows, Hooded Crows, and Ravens.
For instance, Mr. Ward Fowler records a case in which a pair of Crows
attacked a small rookery, ransacked the nests, and destroyed the eggs,
with the result that not a single pair of Rooks was left in the
settlement. Each Rook must therefore secure a position within the
precincts of the community if it is to have a chance of success in the
attainment of reproduction. But every locality cannot supply sufficient
trees of the right kind, appropriately situated and in suitable relation
to the food supply, in which numbers of nests can be built i