lity to do all this that is demanded of them will be severely taxed
by the brooding which must perforce curtail the time available for the
collection of food.

Let us then suppose that the Reed-Buntings inhabiting a certain piece of
marsh are divided into two classes, those which are pugnacious and
intolerant of the approach of strangers, and those which welcome their
presence. The nests of the former will be built in isolation, those of
the latter in close proximity. In due course eggs will be laid and
incubation performed, and thus far all alike will probably be
successful. Here, however, a critical point is reached. If the young are
to be freed from the risk of exposure, the parents must find the
necessary supply of food rapidly. But manifestly all will not be in a
like satisfactory position to accomplish this, for whereas the isolated
pairs will have free access to all the food in the immediate vicinity of
the nest, those which have built in proximity to one another, meeting
competition in every direction, will be compelled to roam farther
afield and waste much valuable time by doing so; and under conditions
which can well be imagined, even this slight loss of time will be
sufficient to impede the growth of the delicate offspring, or to lead
perhaps to still greater disaster. If any one doubts this, let him first
examine one of the fragile offspring; let him then study the conditions
under which it is reared, observing the proportion of time it passes in
sleep and the anxiety of the parent bird to brood; and finally let him
picture to himself its plight in a wet season if, in order to collect
the necessary food, the parents were obliged to absent themselves for
periods of long duration.

Now take the case of the Guillemot. Its young at birth are by no means
helpless in the sense that the young Reed-Bunting is, and food is
readily procured. But breeding stations are scarce, for although there
are many miles of cliff-bound coast, yet not every type of rock
formation produces the fissures and ledges upon which the bird rests.
Hence vast stretches of coast-line remain uninhabited, and the birds are
forced to concentrate at certain points, where year after year they
assemble in countless numbers from distant parts of the ocean. If, then,
different individuals were to jostle one another from adjoining
positions, and each one were to attempt to occupy a ledge in solitary
State, not only would the successful ones gain no a