of suitable breeding
ground, no greater calamity could befall the species than that some
members should exercise dominion over too large an area of the habitable
part of the cliff and thus prevent others from breeding. Under such
conditions the race could not endure, since in this, as in every case,
its survival must depend upon a close correspondence between the
behaviour of the individual and the circumstances in the external
environment.

Scarcity of suitable cliffs is the principal reason of the ledges being
so closely packed with Guillemots, just as it accounts for this part of
the precipice being crowded with Kittiwake Gulls, that part with
Herring-Gulls, and that part again with Razorbills and Puffins. Yet each
individual preserves its few square feet of rock or soil from
molestation, and the area each one occupies varies according to the
conditions of existence of the species. Thus the Herring-Gull occupies a
comparatively small area, although one many times larger than that of
the Guillemot. It requires more space than the latter, owing to the fact
that it not only builds a nest but rears four instead of a single
offspring, and it can be allowed this, because, since its young remain
in the nest until they are capable of sustained flight, it can make use
of many miles of cliff from which the tide recedes at the base, or which
have, at their base, rocks jutting out into the sea; but manifestly it
cannot be allowed so much space as the Bunting.

Martins build in close proximity to one another, owing probably to
shortage of accommodation, and, in their case, the nests have to be so
situated as to be sheltered from the wet. If water drips upon them for
any length of time, the mud, of which they are composed, crumbles and
large pieces fall away, with the result that the eggs or the young are
precipitated to the ground. Consequently, not every house or
perpendicular cliff will answer the purpose of a breeding station. A few
pairs build their nests beneath the eaves close against the walls of my
house, and year after year the result is much the same; after every
downfall of rain, the water collects into rivulets, trickles down over
the eaves, is absorbed by the mud and destroys the nests. Thereupon, the
birds set to work and rebuild; but again the nest is destroyed, and
again they rebuild, and so on throughout the summer, and only on rare
occasions do they succeed in rearing offspring at the proper season.
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