.
Why is this bird so extraordinarily abundant, while others producing two
or three times as many young are much less plentiful? The explanation is
not difficult. The food most congenial to this species, and on which it
thrives best, is abundantly distributed over a very extensive region,
offering such differences of soil and climate, that in one part or
another of the area the supply never fails. The bird is capable of a
very rapid and long-continued flight, so that it can pass without
fatigue over the whole of the district it inhabits, and as soon as the
supply of food begins to fail in one place is able to discover a fresh
feeding-ground. This example strikingly shows us that the procuring a
constant supply of wholesome food is almost the sole condition requisite
for ensuring the rapid increase of a given species, since neither the
limited fecundity, nor the unrestrained attacks of birds of prey and of
man are here sufficient to check it. In no other birds are these
peculiar circumstances so strikingly combined. Either their food is more
liable to failure, or they have not sufficient power of wing to search
for it over an extensive area, or during some season of the year it
becomes very scarce, and less wholesome substitutes have to be found;
and thus, though more fertile in offspring, they can never increase
beyond the supply of food in the least favourable seasons.

Many birds can only exist by migrating, when their food becomes scarce,
to regions possessing a milder, or at least a different climate, though,
as these migrating birds are seldom excessively abundant, it is evident
that the countries they visit are still deficient in a constant and
abundant supply of wholesome food. Those whose organization does not
permit them to migrate when their food becomes periodically scarce, can
never attain a large population. This is probably the reasons why
woodpeckers are scarce with us, while in the tropics they are among the
most abundant of solitary birds. Thus the house sparrow is more
abundant than the redbreast, because its food is more constant and
plentiful,--seeds of grasses being preserved during the winter, and our
farm-yards and stubble-fields furnishing an almost inexhaustible supply.
Why, as a general rule, are aquatic, and especially sea birds, very
numerous in individuals? Not because they are more prolific than others,
generally the contrary; but because their food never fails, the
sea-shores and river-ban