h control.

Some birds, however, have no difficulty in finding the necessary food
for their young, yet have great difficulty in finding a station where
they can rear their young in safety; and the area each one occupies has
been reduced to the smallest proportions in order that the maximum
number can be accommodated. Here, any increase in the size of the
territory would inevitably lead to the extinction of the race, so that
nothing stands between failure and success except the ability of the
bird to defend its territory. If we study the bird population at one of
the breeding stations on the coast, we find, generally speaking, that
each kind of bird inhabits a particular portion of the cliff; on the
lower ledges are the Guillemots and Kittiwake Gulls; higher up are
Razorbills and Fulmars, and at the top, where the cliff is broken and
the face of the rock covered with turf and soil, the Puffin finds
shelter for its egg. At the same time there is much overlapping; the
kind of ledge that suits a Razorbill is equally suitable for a Guillemot
or a Fulmar, and so, no matter how successful the Razorbill may be in
establishing a territory and preventing intrusion upon it by other
Razorbills, it will be all to no purpose if it allows itself to be
jostled out of its position by a Fulmar. Hence, inasmuch as breeding
stations are limited and competition for territory so severe, only those
forms in which the fighting instinct responds freely to a wide range of
stimuli will be in a position to maintain a footing upon the cliff.

In trying to estimate the importance of the hostility in its relation to
the territory, we must bear in mind that competition varies in different
seasons and in different localities. The surface of the land is
constantly undergoing modification, partly owing to human and partly to
physical agency--forests are cleared; marshes are drained; the face of
the sea-cliffs is altered by the erosion of the waves; here the coast
may be locally elevated, there locally depressed; and so forth. Many of
these changes are slow and imperceptible, many can be observed in our
own lifetime. The timber is felled and the undergrowth cleared in some
wood, and the following spring we notice a change in the character of
the bird population. Migrants which formerly found in it no suitable
accommodation now begin to appear, and as the seasons pass by and the
undergrowth affords more and more shelter for the nests and an
increasing sup